UT Testing-Self Study Notes 20140807 PDF [PDF]

Zitiervorschau

Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT 2014-July Facilitators: Fion Zhang/ Charliechong

http://en.wikipedia.org/wiki/Greek_alphabet

Numerical Prefix  Micro - (µ) a prefix in the SI and other systems of units denoting a factor of 10-6 (one millionth)  Nano - a prefix in the SI and other systems of units denoting a factor of 10-9 (one billionth)  Pico - a prefix in the International System of Units (SI) denoting a factor of 10-12

Speaker: Fion Zhang 2014/July/31

http://meilishouxihu.blog.163.com/

Contents: 1. ASNT Level III Exam Topical Outline 2. AE Codes and Standards ■ ASTM. ■ ASME V. 3. Reading 01 Introduction to UT by ndt-ed.org with thanks (always) 1. 2. 3. 4. 5. 6.

Others reading. Addendum 1 – Equipment Calibrations Addendum 2 – Equations & Calculations. Addendum 3 – Questions & Answers Addendum 4 – Questions & Answers Part Calculations Addendum 5 – Questions & Answers Level I, II, III

ASNT UT Level III Examination Topical Outline This examination is 4 hours in length, having 135 questions of equal value. 1. Principles/Theory 2. Equipment/Materials 3. Techniques/Calibrations • Contact • Immersion • Comparison of contact and immersion methods • Remote monitoring • Calibration (electronic and functional)

https://www.asnt.org/MajorSiteSections/Certification/ASNT%20NDT%20Level%20I II%20Program/NDT%20Level%20III%20Examinations

4. Interpretation/Evaluations • Evaluation of base metal product forms • Evaluation of weldments • Evaluation of bonded structures • Variables affecting test results • Evaluation (general) 5. Procedures • Specific applications • Codes/Standards/Specifications 6. Safety and Health

References 1. Level III Study Guide: Ultrasonic Testing (2261) 2. NDT Handbook: Volume 7, Ultrasonic Testing (147) 3. Supplement to Recommended Practice No. SNT-TC-1A (Q&A Book) Ultrasonic Testing Method (2028) 4. Ultrasonics: Fundamentals, Technology, Applications (341) 5. Refresher Course: ASNT offers a UT Refresher Course based on the Body of Knowledge outlined above. The number in parentheses following each reference is the ASNT catalog number.

UT - Ultrasonic Testing Length: 4 hours Questions: 135 1. Principles/Theory • Nature of sound waves • Modes of sound wave generation • Velocity, frequency, and wavelength of sound waves • Attenuation of sound waves • Acoustic impedance • Reflection • Refraction and mode conversion • Snell’s law and critical angles • Fresnel and Fraunhofer effects

2. Equipment/Materials • Pulse/echo instrumentation • Digital thickness instrumentation • Transducer operation and theory • Transducer operation/manipulations • Resonance testing equipment • Couplants • Calibration blocks • Cables/connectors • Test specimen • Miscellaneous materials

3. Techniques/Calibrations • Contact • Immersion • Comparison of contact and immersion methods • Remote monitoring • Calibration (electronic and functional)

4. Interpretation/Evaluations • Evaluation of base metal product forms • Evaluation of weldments • Evaluation of bonded structures • Variables affecting test results • Evaluation (general) 5. Procedures • Specific applications • Codes/Standards/Specifications Reference Catalog Number NDT Handbook, Second Edition: Volume 7, Ultrasonic Testing 132 ASNT Level III Study Guide: Ultrasonic Testing 2261A Ultrasonics: Fundamentals, Technology, Applications 341

ASME V Article Numbers: Gen RT Nil UT UT PT MT ET Visual LT AE Qualif. ACFM

Article 1 Article 2 Article 3 Article 4 for welds Article 5 for materials Article 6 Article 7 Article 8 Article 9 Article 10 Article 11 (FRP) /Article 12 (Metallic) / Article 13 (Continuous) Article 14 Article 15

ASTM/ AWS Standards • ASTM E494 – 10: Practice for Measuring Ultrasonic Velocity in Materials. • ASTM standard E-164, "Standard Practice for Contact Examination of Weldments“. • AWS Structural Welding Code, section 6. • Annual Book of the American Society of Testing and Materials, ASTM. Volume 03.03, Nondestructive Testing

Other Reading • •

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http://techcorr.com/services/Inspection-and-Testing/Ultrasonic-Shear-Wave.cfm http://www.cnde.iastate.edu/faacasr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Ultrasonic%20Pages/Ultraprinciples.html http://www.ndt.net/article/v05n09/berke/berke1.htm#0 http://www.mie.utoronto.ca/labs/undel/index.php?menu_path=menu_pages/projects_menu.htm l&content_path=content_pages/fac2_2.html&main_menu=projects&side_menu=page1&sub_si de_menu=s2 https://www.nde-ed.org/GeneralResources/Glossary/letter/d.htm

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http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/general/ http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/ http://www.olympus-ims.com/en/knowledge/

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http://wenku.baidu.com/view/3cf257781711cc7931b716e0.html http://www.docin.com/p-148566003.html http://www.studyblue.com/notes/note/n/ut-asnt-level-ii/deck/6278710

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Study Note 1: Ultrasonic Testing

Source: http://www.ndted.org/EducationResources/CommunityCollege/Ultra sonics/cc_ut_index.htm

Content: Section 1: Introduction 1.1: Basic Principles of Ultrasonic Testing 1.2: Advantages and Disadvantages 1.3: Limitations

Content: Section 2: Physics of Ultrasound 2.1: Wave Propagation 2.2: Modes of Sound Wave Propagation 2.3: Properties of Acoustic Plane Wave 2.4: Wavelength and Defect Detection 2.5: Sound Propagation in Elastic Materials 2.6: Attenuation of Sound Waves 2.7: Acoustic Impedance 2.8: Reflection and Transmission Coefficients (Pressure) 2.9: Refraction and Snell's Law 2.10: Mode Conversion 2.11: Signal-to-Noise Ratio 2.12: Wave Interaction or Interference 2.13: Inverse Square Rule/ Inverse Rule 2.14: Resonance 2.15 Measurement of Sound 2.16 Practice Makes Perfect

Content: Section 3: Equipment & Transducers 3.1: Piezoelectric Transducers 3.2: Characteristics of Piezoelectric Transducers 3.3: Radiated Fields of Ultrasonic Transducers 3.4: Transducer Beam Spread 3.5: Transducer Types 3.6: Transducer Testing I 3.7: Transducer Testing II 3.8: Transducer Modeling 3.9: Couplants 3.10: Electromagnetic Acoustic Transducers (EMATs) Continues Next Page

3.11: Pulser-Receivers 3.12: Tone Burst Generators In Research 3.13: Arbitrary Function Generators 3.14: Electrical Impedance Matching and Termination 3.15: Data Presentation 3.16: Error Analysis 3.17: Transducer Quality Factor “Q” 3.18: Testing Techniques 3.19: UT Equipment Circuitry 3.20: Further Reading on Sub-Section 3

Content: Section 4: Calibration Methods 4.1: Calibration Methods 4.2: The Calibrations 4.2.1: Distance Amplitude Correction (DAC) 4.2.2: Finding the probe index 4.2.3: Checking the probe angle 4.2.4: Calibration of shear waves for range V1 Block 4.2.5: Dead Zone 4.2.7: Transfer Correction 4.2.8: Linearity Checks (Time Base/ Equipment Gain/ Vertical Gain) 4.2.9: TCG-Time Correction Gain 4.3: Curvature Correction 4.4: Calibration References & Standards 4.5: Exercises 4.6: Video Time

Content: Section 5: Measurement Techniques 5.1: Normal Beam Inspection 5.2: Angle Beams 5.3: Reflector Sizing 5.4: Automated Scanning 5.5: Precision Velocity Measurements 5.6: Attenuation Measurements 5.7: Spread Spectrum Ultrasonics 5.8: Signal Processing Techniques 5.9: Scanning Methods 5.10: Scanning Patterns 5.11: Pulse Repetition Rate and Penetration 5.12: Interferences & Non Relevant Indications 5.13: Entry Surface Variables 5.14: The Concept of Effective Distance 5.15: Exercises

Content: Section 6: Selected Applications & Techniques 6.1: Defects & Discontinuities 6.2: Rail Inspection 6.3: Weldments (Welded Joints) 6.4: Pipe & Tube 6.5: Echo Dynamic 6.6: Technique Sheets 6.7: Material Properties-Elastic Modulus Measurements 6.8: High Temperature Ultrasonic Testing 6.9: TOFD Introduction

Content: Section 7: Reference Material 7.1: UT Material Properties 7.2: General References & Resources 7.3: Video Time Content: Section 8: Ultrasonic Inspection Quizzes 8.1: Ultrasonic Inspection Quizzes 8.2: Online UT Quizzes

Section 1: Introduction

1.1:

Basic Principles of Ultrasonic Testing

ULTRASONIC INSPECTION is a nondestructive method in which beams of high-frequency sound waves are introduced into materials for the detection of surface and subsurface flaws in the material. The sound waves travel through the material with some attendant loss of energy (attenuation) and are reflected at interfaces. The reflected beam is displayed and then analyzed to define the presence and location of flaws or discontinuities. The degree of reflection depends largely on the physical state of the materials forming the interface and to a lesser extent on the specific physical properties of the material.

For example, sound waves are almost completely reflected at metal/gas interfaces. Partial reflection occurs at metal/liquid or metal/solid interfaces, with the specific percentage of reflected energy depending mainly on the ratios of certain properties of the material on opposing sides of the interface. Cracks, laminations, shrinkage cavities, bursts, flakes, pores, disbonds, and other discontinuities that produce reflective interfaces can be easily detected. Inclusions and other in-homogeneities can also be detected by causing partial reflection or scattering of the ultrasonic waves or by producing some other detectable effect on the ultrasonic waves.

In ultrasonic testing, the reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when a echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/ultrasoundInspection.swf

Basics of Ultrasonic Test- Contact Pulse Echo Method

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Ultrasonic%20Pages/Ultra-principles.html

Immersion Method- Figure below shows an immersion UT setup with CRT or computer screen display. IP indicates the initial pulse while FW and BW indicate the front and back wall of the specimen, respectively.

Amplitude

Display / CRT

Water path

Time / Distance

Basics of Ultrasonic Test- A-Scan

1.2: Source-1: The advantages of ultrasonic testing include Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include:  It is sensitive to both surface and subsurface discontinuities.  The depth of penetration for flaw detection or measurement is superior to other NDT methods.  Only single-sided access is needed when the pulse-echo technique is used.  It is highly accurate in determining reflector position and estimating size and shape.  Minimal part preparation is required.  Electronic equipment provides instantaneous results.  Detailed images can be produced with automated systems.  It has other uses, such as thickness measurement, in addition to flaw detection.

Source-2: The advantages of ultrasonic testing include     

It can be used to determine mechanical properties and microstructure. It can be used for imaging and microscopy. It is portable and cost effective. It can be used with all states of matter except plasma and vacuum. It is not affected by optical density.

Source-3: Advantages and Disadvantages The principal advantages of ultrasonic inspection as compared to other methods for nondestructive inspection of metal parts are:  Superior penetrating power, which allows the detection of flaws deep in the part. Ultrasonic inspection is done routinely to thicknesses of a few meters on many types of parts and to thicknesses of about 6 m (20 ft) in the axial inspection of parts such as long steel shafts or rotor forgings  High sensitivity, permitting the detection of extremely small flaws  Greater accuracy than other nondestructive methods in determining the position of internal flaws, estimating their size, and characterizing their orientation, shape, and nature  Only one surface needs to be accessible

 Operation is electronic, which provides almost instantaneous indications of flaws. This makes the method suitable for immediate interpretation, automation, rapid scanning, in-line production monitoring, and process control. With most systems, a permanent record of inspection results can be made for future reference  Volumetric scanning ability, enabling the inspection of a volume of metal extending from front surface to back surface of a part  Nonhazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity  Portability  Provides an output that can be processed digitally by a computer to characterize defects and to determine material properties

The disadvantages of ultrasonic inspection include the following:  Manual operation requires careful attention by experienced technicians.  Extensive technical knowledge is required for the development of inspection procedures.  Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect.  Discontinuities that are present in a shallow layer immediately beneath the surface may not be detectable.  Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected.  Reference standards are needed, both for calibrating the equipment and for characterizing flaws.

1.3:

Limitations (Disadvantages)

As with all NDT methods, ultrasonic inspection also has its limitations, which include: • Surface must be accessible to transmit ultrasound. • Skill and training is more extensive than with some other methods. • It normally requires a coupling medium to promote the transfer of sound energy into the test specimen. • Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect. • Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise. • Linear defects oriented parallel to the sound beam may go undetected. • Reference standards are required for both equipment calibration and the characterization of flaws.

Section 2: Physics of Ultrasound

Content: Section 2: Physics of Ultrasound 2.0: Ultrasound Formula 2.1: Wave Propagation 2.2: Modes of Sound Wave Propagation 2.3: Sound Propagation in Elastic Materials 2.4: Properties of Acoustic Plane Wave 2.5: Wavelength and Defect Detection 2.6: Attenuation of Sound Waves 2.7: Acoustic Impedance 2.8: Reflection and Transmission Coefficients (Pressure) 2.9: Refraction and Snell's Law 2.10: Mode Conversion 2.11: Signal-to-Noise Ratio 2.12: The Sound Fields- Dead / Fresnel & Fraunhofer Zones 2.13: Inverse Square Rule/ Inverse Rule 2.14: Resonance 2.15 Measurement of Sound 2.16 Practice Makes Perfect

2.0: Ultrasound Formula

http://www.ndt-ed.org/GeneralResources/Calculator/calculator.htm

Ultrasonic Formula

Ultrasonic Formula

Parameters of Ultrasonic Waves

2.1: Wave Propagation Ultrasonic testing is based on time-varying deformations or vibrations in materials, which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibration motion about their equilibrium positions. Many different patterns of vibration motion exist at the atomic level, however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in unison to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal (electrostatic) restoration forces arise. It is these elastic restoring forces between particles, combined with inertia of the particles, that leads to the oscillatory motions of the medium. Keywords: ■ internal (electrostatic) restoration forces ■ inertia of the particles

Acoustic Spectrum

Acoustic Spectrum

Acoustic Spectrum

Acoustic Wave – Node and Anti-Node The points where the two waves constantly cancel each other are called nodes, and the points of maximum amplitude between them, antinodes.

http://www.physicsclassroom.com/Class/waves/u10l4c.cfm http://www.physicsclassroom.com/Class/waves/h4.gif

Acoustic Wave – Node and Anti-Node Formation of a standing wave by two waves from opposite directions

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/standw.html

Q151 A point, line or surface of a vibration body marked by absolute or relative freedom from vibratory motion (momentarily?) is referred to as: a) b) c) d)

a node an antinode rarefaction compression

2.2: Modes of Sound Wave Propagation 2.2.1 Modes of Ultrasound In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as;    

longitudinal waves, shear waves, surface waves, and in thin materials as plate waves.

Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below.

2.2.2

Propagation & Polarization Vectors

 Propagation Vector- The direction of wave propagation  Polarization Vector- The direction of particle motion

Longitudinal and shear waves

Longitudinal and shear waves- Defined the Vectors

Longitudinal and shear waves

Longitudinal and shear waves

2.2.3 Longitudinal Wave Also Knows as:    

longitudinal waves, pressure wave compressional waves. density waves

can be generated in (1) liquids, as well as (2) solids

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/longitudinal.swf

In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of compressions and expansion (rarefaction) movements.

Longitudinal wave: Longitudinal waves (L-Waves) compress and decompress the material in the direction of motion, much like sound waves in air.

Longitudinal Wave

2.2.4

Shear waves (S-Waves)

In air, sound travels by the compression and rarefaction of air molecules in the direction of travel. However, in solids, molecules can support vibrations in other directions, hence, a number of different types of sound waves are possible. Waves can be characterized in space by oscillatory patterns that are capable of maintaining their shape and propagating in a stable manner. The propagation of waves is often described in terms of what are called “wave modes.” As mentioned previously, longitudinal and transverse (shear) waves are most often used in ultrasonic inspection. However, at surfaces and interfaces, various types of elliptical or complex vibrations of the particles make other waves possible. Some of these wave modes such as (1) Rayleigh and (2) Lamb waves are also useful for ultrasonic inspection. Keywords: Compression Rarefaction

Shear waves vibrate particles at right angles compared to the motion of the ultrasonic wave. The velocity of shear waves through a material is approximately half that of the longitudinal waves. The angle in which the ultrasonic wave enters the material determines whether longitudinal, shear, or both waves are produced.

Shear waves

In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/transverse.swf

Q10: For a shear wave travelling from steel to water incident on the boundary at 10 degrees will give a refracted shear wave in water with an angle of: A. B. C. D.

0 degrees 5 degrees 20 degrees none of the above

2.2.5

Rayleigh Characteristics

Rayleigh waves are a type of surface wave that travel near the surface of solids. Rayleigh waves include both longitudinal and transverse motions that decrease exponentially in amplitude as distance from the surface increases. There is a phase difference between these component motions. In isotropic solids these waves cause the surface particles to move in ellipses in planes normal to the surface and parallel to the direction of propagation – the major axis of the ellipse is vertical. At the surface and at shallow depths this motion is retrograde 逆行, that is the in-plane motion of a particle is counterclockwise when the wave travels from left to right. http://en.wikipedia.org/wiki/Rayleigh_wave

Rayleigh waves are a type of surface acoustic wave that travel on solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in nondestructive testing for detecting defects. They are part of the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves.

Rayleigh waves

Q29: The longitudinal wave incident angle which results in formation of a Rayleigh wave is called: A. B. C. D.

Normal incidence The first critical angle The second critical angle Any angle above the first critical angle

Surface (or Rayleigh) waves travel the surface of a relatively thick solid material penetrating to a depth of one wavelength. Surface waves combine both (1) a longitudinal and (2) transverse motion to create an elliptic orbit motion as shown in the image and animation below.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/rayleigh.swf

The major axis of the ellipse is perpendicular to the surface of the solid. As the depth of an individual atom from the surface increases the width of its elliptical motion decreases. Surface waves are generated when a longitudinal wave intersects a surface near the second critical angle and they travel at a velocity between .87 and .95 of a shear wave. Rayleigh waves are useful because they are very sensitive to surface defects (and other surface features) and they follow the surface around curves. Because of this, Rayleigh waves can be used to inspect areas that other waves might have difficulty reaching. Wave velocity:  Longitudinal wave velocity =1v,  The velocity of shear waves through a material is approximately half that of the longitudinal waves, (≈0.5v)  Surface waves are generated when a longitudinal wave intersects a surface near the second critical angle and they travel at a velocity between .87 and .95 of a shear wave. ≈(0.87~0.95)x0.5v

The major axis of the ellipse is perpendicular to the surface of the solid.

Surface wave

Surface wave or Rayleigh wave are formed when shear waves refract to 90. The whip-like particle vibration of the shear wave is converted into elliptical motion by the particle changing direction at the interface with the surface. The wave are not often used in industrial NDT although they do have some application in aerospace industry. Their mode of propagation is elliptical along the surface of material, penetrating to a depth of one wavelength. They will follow the contour of the surface and they travel at approximately 90% of the velocity of the shear waves. Depth of penetration of about one wavelength

Direction of wave propagation

Surface wave has the ability to follow surface contour, until it meet a sharp change i.e. a surface crack/seam/lap. However the surface waves could be easily completely absorbed by excess couplant of simply touching the part ahead of the waves.

Transducer Wedge

Surface discontinuity

Specimen

Surface wave - Following Contour

Surface wave

Surface wave – One wavelength deep

λ

λ

Rayleigh Wave

http://web.ics.purdue.edu/~braile/edumod/waves/Rwave_files/image001.gif

Love Wave

http://web.ics.purdue.edu/~braile/edumod/waves/Lwave_files/image001.gif

Love Wave

Other Reading: Rayleigh Waves Surface waves (Rayleigh waves) are another type of ultrasonic wave used in the inspection of materials. These waves travel along the flat or curved surface of relatively thick solid parts. For the propagation of waves of this type, the waves must be traveling along an interface bounded on one side by the strong elastic forces of a solid and on the other side by the practically negligible elastic forces between gas molecules. Surface waves leak energy into liquid couplants and do not exist for any significant distance along the surface of a solid immersed in a liquid, unless the liquid covers the solid surface only as a very thin film. Surface waves are subject to attenuation in a given material, as are longitudinal or transverse waves. They have a velocity approximately 90% of the transverse wave velocity in the same material. The region within which these waves propagate with effective energy is not much thicker than about one wavelength beneath the surface of the metal.

At this depth, wave energy is about 4% of the wave energy at the surface, and the amplitude of oscillation decreases sharply to a negligible value at greater depths. Surface waves follow contoured surfaces. For example, surface waves traveling on the top surface of a metal block are reflected from a sharp edge, but if the edge is rounded off, the waves continue down the side face and are reflected at the lower edge, returning to the sending point. Surface waves will travel completely around a cube if all edges of the cube are rounded off. Surface waves can be used to inspect parts that have complex contours.

Q110: What kind of wave mode travel at a velocity slightly below the shear wave and their modes of propagation are both longitudinal and transverse with respect to the surface? a) b) c) d)

Rayleigh wave Transverse wave L-wave Longitudinal wave

Q: Which of the following modes of vibration exhibits the shortest wavelength at a given frequency and in a given material? A. B. C. D.

longitudinal wave compression wave shear wave surface wave

2.2.6

Lamb Wave:

Lamb waves propagate in solid plates. They are elastic waves whose particle motion lies in the plane that contains the direction of wave propagation and the plate normal (the direction perpendicular to the plate). In 1917, the english mathematician horace lamb published his classic analysis and description of acoustic waves of this type. Their properties turned out to be quite complex. An infinite medium supports just two wave modes traveling at unique velocities; but plates support two infinite sets of lamb wave modes, whose velocities depend on the relationship between wavelength and plate thickness.

Since the 1990s, the understanding and utilization of lamb waves has advanced greatly, thanks to the rapid increase in the availability of computing power. Lamb's theoretical formulations have found substantial practical application, especially in the field of nondestructive testing. The term rayleigh–lamb waves embraces the rayleigh wave, a type of wave that propagates along a single surface. Both rayleigh and lamb waves are constrained by the elastic properties of the surface(s) that guide them. http://en.wikipedia.org/wiki/Lamb_wave http://pediaview.com/openpedia/Lamb_waves

Types of Wave

New!  Plate wave- Love  Stoneley wave  Sezawa

Plate or Lamb waves are the most commonly used plate waves in NDT. Lamb waves are complex vibrational waves that propagate parallel to the test surface throughout the thickness of the material. Propagation of Lamb waves depends on the density and the elastic material properties of a component. They are also influenced a great deal by the test frequency and material thickness. Lamb waves are generated at an incident angle in which the parallel component of the velocity of the wave in the source is equal to the velocity of the wave in the test material. Lamb waves will travel several meters in steel and so are useful to scan plate, wire, and tubes. Lamb wave influenced by: (Dispersive Wave) ■ ■ ■ ■

Density Elastic material properties Frequencies Material thickness

Plate or Lamb waves are similar to surface waves except they can only be generated in materials a few wavelengths thick.

http://www.ndt.net/ndtaz/files/lamb_a.gif

Plate wave or Lamb wave are formed by the introduction of surface wave into a thin material. They are a combination of (1) compression and surface or (2) shear and surface waves causing the plate material to flex by totally saturating the material. The two types of plate waves:

With Lamb waves, a number of modes of particle vibration are possible, but the two most common are symmetrical and asymmetrical. The complex motion of the particles is similar to the elliptical orbits for surface waves. Symmetrical Lamb waves move in a symmetrical fashion about the median plane of the plate. This is sometimes called the extensional mode because the wave is “stretching and compressing” the plate in the wave motion direction. Wave motion in the symmetrical mode is most efficiently produced when the exciting force is parallel to the plate. The asymmetrical Lamb wave mode is often called the “flexural mode” because a large portion of the motion moves in a normal direction to the plate, and a little motion occurs in the direction parallel to the plate. In this mode, the body of the plate bends as the two surfaces move in the same direction. The generation of waves using both piezoelectric transducers and electromagnetic acoustic transducers (EMATs) are discussed in later sections. Keywords: Symmetrical = extensional mode Asymmetrical = flexural mode

When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves. Lamb waves – 2 modes

Symmetrical = extensional mode Asymmetrical = flexural mode

Symmetrical = extensional mode Asymmetrical = flexural mode

Symmetrical = extensional mode

Other Reading: Lamb Wave Lamb waves, also known as plate waves, are another type of ultrasonic wave used in the nondestructive inspection of materials. Lamb waves are propagated in plates (made of composites or metals) only a few wavelengths thick. A Lamb wave consists of a complex vibration that occurs throughout the thickness of the material. The propagation characteristics of Lamb waves depend on the density, elastic properties, and structure of the material as well as the thickness of the test piece and the frequency. Their behavior in general resembles that observed in the transmission of electromagnetic waves through waveguides. There are two basic forms of Lamb waves:  Symmetrical, or dilatational  Asymmetrical, or bending

The form is determined by whether the particle motion is symmetrical or asymmetrical with respect to the neutral axis of the test piece. Each form is further subdivided into several modes having different velocities, which can be controlled by the angle at which the waves enter the test piece. Theoretically, there are an infinite number of specific velocities at which Lamb waves can travel in a given material. Within a given plate, the specific velocities for Lamb waves are complex functions of plate thickness and frequency. In symmetrical (dilatational) Lamb waves, there is a compressional (longitudinal) particle displacement along the neutral axis of the plate and an elliptical particle displacement on each surface (Fig. 4a). In asymmetrical (bending) Lamb waves, there is a shear (transverse) particle displacement along the neutral axis of the plate and an elliptical particle displacement on each surface (Fig. 4b). The ratio of the major to minor axes of the ellipse is a function of the material in which the wave is being propagated.

Fig. 4 Diagram of the basic patterns of (a) symmetrical (dilatational) and (b) asymmetrical (bending) Lamb waves. The wavelength, , is the distance corresponding to one complete cycle.

Q1: The wave mode that has multiple or varying wave velocities is: A. B. C. D.

Longitudinal waves Shear waves Transverse waves Lamb waves

2.2.7

Dispersive Wave:

Wave modes such as those found in Lamb wave have a velocity of propagation dependent upon the operating frequency, sample thickness and elastic moduli. They are dispersive (velocity change with frequency) in that pulses transmitted in these mode tend to become stretched or dispersed.

Dispersion refers to the fact that in a real medium such as water, air, or glass, a wave traveling through that medium will have a velocity that depends upon its frequency. Dispersion occurs for any form of wave, acoustic, electromagnetic, electronic, even quantum mechanical. Dispersion is responsible for a prism being able to resolve light into colors and defines the maximum frequency of broadband pulses one can send down an optical fiber or through a copper wire. Dispersion affects wave and swell forecasts at sea and influences the design of sound equipment. Dispersion is a physical property of the medium and can combine with other properties to yield very strange results. For example, in the propagation of light in an optical fiber, the glass introduces dispersion and separates the wavelengths of light according to frequency, however if the light is intense enough, it can interact with the electrons in the material changing its refractive index. The combination of dispersion and index change can cancel each other leading to a wave that can propagate indefinitely maintaining a constant shape. Such a wave has been termed a soliton. http://www.rpi.edu/dept/chem-eng/WWW/faculty/plawsky/Comsol%20Modules/DispersiveWave/DispersiveWave.html

Plate or Lamb waves are generated at an incident angle in which the parallel component of the velocity of the wave in the source is equal to the velocity of the wave in the test material.

Thickness Limitation: One can not generate shear / surface (or Lamb?) wave on a plate that is thinner than ½ the wavelength.

2.3: Sound Propagation in Elastic Materials In the previous pages, it was pointed out that sound waves propagate due to the vibrations or oscillatory motions of particles within a material. An ultrasonic wave may be visualized as an infinite number of oscillating masses or particles connected by means of elastic springs. Each individual particle is influenced by the motion of its nearest neighbor and both (1) inertial and (2) elastic restoring forces act upon each particle. A mass on a spring has a single resonant frequency determined by its spring constant k and its mass m. The spring constant is the restoring force of a spring per unit of length. Within the elastic limit of any material, there is a linear relationship between the displacement of a particle and the force attempting to restore the particle to its equilibrium position. This linear dependency is described by Hooke's Law.

Spring model- A mass on a spring has a single resonant frequency determined by its spring constant k and its mass m.

Spring model- A mass on a spring has a single resonant frequency determined by its spring constant k and its mass m.

In terms of the spring model, Hooke's Law says that the restoring force due to a spring is proportional to the length that the spring is stretched, and acts in the opposite direction. Mathematically, Hooke's Law is written as F =-kx, where F is the force, k is the spring constant, and x is the amount of particle displacement. Hooke's law is represented graphically it the bottom. Please note that the spring is applying a force to the particle that is equal and opposite to the force pulling down on the particle.

Elastic Model

Elastic Model / Longitudinal Wave

Elastic Model / Longitudinal Wave

Elastic Model / Shear Wave

Elastic Model / Shear Wave

The Speed of Sound Hooke's Law, when used along with Newton's Second Law, can explain a few things about the speed of sound. The speed of sound within a material is a function of the properties of the material and is independent of the amplitude of the sound wave. Newton's Second Law says that the force applied to a particle will be balanced by the particle's mass and the acceleration of the particle. Mathematically, Newton's Second Law is written as F = ma. Hooke's Law then says that this force will be balanced by a force in the opposite direction that is dependent on the amount of displacement and the spring constant (F = -kx). Therefore, since the applied force and the restoring force are equal, ma = -kx can be written. The negative sign indicates that the force is in the opposite direction.

F= ma = -kx

Since the mass m and the spring constant k are constants for any given material, it can be seen that the acceleration a and the displacement x are the only variables. It can also be seen that they are directly proportional. For instance, if the displacement of the particle increases, so does its acceleration. It turns out that the time that it takes a particle to move and return to its equilibrium position is independent of the force applied. So, within a given material, sound always travels at the same speed no matter how much force is applied when other variables, such as temperature, are held constant.

a∝x

What properties of material affect its speed of sound? Of course, sound does travel at different speeds in different materials. This is because the (1) mass of the atomic particles and the (2) spring constants are different for different materials. The mass of the particles is related to the density of the material, and the spring constant is related to the elastic constants of a material. The general relationship between the speed of sound in a solid and its density and elastic constants is given by the following equation:

Elastic constant → spring constants

Density → mass of the atomic particles

Where V is the speed of sound, C is the elastic constant, and p is the material density. This equation may take a number of different forms depending on the type of wave (longitudinal or shear) and which of the elastic constants that are used. The typical elastic constants of a materials include:  Young's Modulus, E: a proportionality constant between uniaxial stress and strain.  Poisson's Ratio, n: the ratio of radial strain to axial strain  Bulk modulus, K: a measure of the incompressibility of a body subjected to hydrostatic pressure.  Shear Modulus, G: also called rigidity, a measure of a substance's resistance to shear.  Lame's Constants, l and m: material constants that are derived from Young's Modulus and Poisson's Ratio.

Q163 Acoustic velocity of materials are primary due to the material's: a) b) c) d)

density elasticity both a and b acoustic impedance

Q50: The principle attributes that determine the differences in ultrasonic velocities among materials are: A. B. C. D.

Frequency and wavelength Thickness and travel time Elasticity and density Chemistry and permeability

When calculating the velocity of a longitudinal wave, Young's Modulus and Poisson's Ratio are commonly used.

When calculating the velocity of a shear wave, the shear modulus is used. It is often most convenient to make the calculations using

Lame's Constants, which are derived from Young's Modulus and Poisson's Ratio.

E/N/G

It must also be mentioned that the subscript ij attached to C (Cij) in the above equation is used to indicate the directionality of the elastic constants with respect to the wave type and direction of wave travel. In isotropic materials, the elastic constants are the same for all directions within the material. However, most materials are anisotropic and the elastic constants differ with each direction. For example, in a piece of rolled aluminum plate, the grains are elongated in one direction and compressed in the others and the elastic constants for the longitudinal direction are different than those for the transverse or short transverse directions.

V longitudinal V transverse

Examples of approximate compressional sound velocities in materials are: Aluminum - 0.632 cm/microsecond 1020 steel - 0.589 cm/microsecond Cast iron - 0.480 cm/microsecond. Examples of approximate shear sound velocities in materials are: Aluminum - 0.313 cm/microsecond 1020 steel - 0.324 cm/microsecond Cast iron - 0.240 cm/microsecond. When comparing compressional and shear velocities, it can be noted that shear velocity is approximately one half that of compressional velocity. The sound velocities for a variety of materials can be found in the ultrasonic properties tables in the general resources section of this site.

Longitudinal Wave Velocity: VL The velocity of a longitudinal wave is described by the following equation:

VL E μ P

= Longitudinal bulk wave velocity = Young’s modulus of elasticity = Poisson ratio = Material density

Shear Wave Velocity: VS The velocity of a shear wave is described by the following equation:

Vs E μ P G

= Shear wave velocity = Young’s modulus of elasticity = Poisson ratio = Material density = Shear modulus

2.4: Properties of Acoustic Plane Wave Wavelength, Frequency and Velocity Among the properties of waves propagating in isotropic solid materials are wavelength, frequency, and velocity. The wavelength is directly proportional to the velocity of the wave and inversely proportional to the frequency of the wave. This relationship is shown by the following equation.

The applet below shows a longitudinal and transverse wave. The direction of wave propagation is from left to right and the movement of the lines indicate the direction of particle oscillation. The equation relating ultrasonic wavelength, frequency, and propagation velocity is included at the bottom of the applet in a reorganized form. The values for the wavelength, frequency, and wave velocity can be adjusted in the dialog boxes to see their effects on the wave. Note that the frequency value must be kept between 0.1 to 1 MHz (one million cycles per second) and the wave velocity must be between 0.1 and 0.7 cm/us.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/applet_2_4/applet_2_4.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/applet_2_4/applet_2_4.htm

Java don’t work? Uninstalled → Reinstalled → Then → http://jingyan.baidu.com/article/9f63fb91d0eab8c8400f0e08.html

Java don’t work? http://jingyan.baidu.com/article/9f63fb91d0eab8c8400f0e08.html

Java don’t work? http://jingyan.baidu.com/article/9f63fb91d0eab8c8400f0e08.html

Java don’t work? http://jingyan.baidu.com/article/9f63fb91d0eab8c8400f0e08.html

Java don’t work? http://jingyan.baidu.com/article/9f63fb91d0eab8c8400f0e08.html

As can be noted by the equation, a change in frequency will result in a change in wavelength. Change the frequency in the applet and view the resultant wavelength. At a frequency of .2 and a material velocity of 0.585 (longitudinal wave in steel) note the resulting wavelength. Adjust the material velocity to 0.480 (longitudinal wave in cast iron) and note the resulting wavelength. Increase the frequency to 0.8 and note the shortened wavelength in each material. In ultrasonic testing, the shorter wavelength resulting from an increase in frequency will usually provide for the detection of smaller discontinuities. This will be discussed more in following sections. Keywords: the shorter wavelength resulting from an increase in frequency will usually provide for the detection of smaller discontinuities

The velocities sound waves The velocities of the various kinds of sound waves can be calculated from the elastic constants of the material concerned, that is the modulus of elasticity E (measured in N/m2), the density p in kg/m3, and Poisson's ratio μ (a dimensionless number). for longitudinal waves:

for transverse waves:

The two velocities of sound are linked by the following relation:

For all solid materials Poisson's ratio μ lies between 0 and 0.5, so that the numerical value of the expression

always lies between 0 and 0.707. In steel and aluminum, μ= 0.28 and 0.34, respectively,

= 0.55 and 0.49 respectIvely. --

2.5: Wavelength and Defect Detection 2.5.1 Sensitivity & Resolution In ultrasonic testing, the inspector must make a decision about the frequency of the transducer that will be used. As we learned on the previous page, changing the frequency when the sound velocity is fixed will result in a change in the wavelength of the sound. The wavelength of the ultrasound used has a significant effect on the probability of detecting a discontinuity. A general rule of thumb is that a discontinuity must be larger than one-half the wavelength to stand a reasonable chance of being detected.

Sensitivity and resolution are two terms that are often used in ultrasonic inspection to describe a technique's ability to locate flaws. Sensitivity is the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths). Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface. Resolution also generally increases as the frequency increases.

Keywords:  Discontinuity must be larger than one-half the wavelength to stand a reasonable chance of being detected.  Sensitivity is the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths).  Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface.  Resolution also generally increases as the frequency increases, pulse length decrease, bandwidth increase (highly damp)

2.5.2

Grain Size & Frequency Selection

The wave frequency can also affect the capability of an inspection in adverse ways. Therefore, selecting the optimal inspection frequency often involves maintaining a balance between the favorable and unfavorable results of the selection. Before selecting an inspection frequency, the material's grain structure and thickness, and the discontinuity's type, size, and probable location should be considered. As frequency increases, sound tends to scatter from large or course grain structure and from small imperfections within a material. Cast materials often have coarse grains and other sound scatters that require lower frequencies to be used for evaluations of these products. (1) Wrought and (2) forged products with directional and refined grain structure can usually be inspected with higher frequency transducers.

Keywords:  Coarse grains →Lower frequency to avoid scattering and noise,  Fine grains →Higher frequency to increase sensitivity & resolution.

Since more things in a material are likely to scatter a portion of the sound energy at higher frequencies, the penetrating power (or the maximum depth in a material that flaws can be located) is also reduced. Frequency also has an effect on the shape of the ultrasonic beam. Beam spread, or the divergence of the beam from the center axis of the transducer, and how it is affected by frequency will be discussed later. It should be mentioned, so as not to be misleading, that a number of other variables will also affect the ability of ultrasound to locate defects. These include the pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter, and the receiver circuitry of the instrument. These are discussed in more detail in the material on signalto-noise ratio.

Coarse grains →Lower frequency to avoid scattering and noise, Fine grains →Higher frequency to increase sensitivity & resolution.

http://www.cnde.iastate.edu/ultrasonics/grain-noise

Detectability variable:      

pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter, and the receiver circuitry of the instrument.

Keywords:  Higher the frequency, greater the scattering, thus less penetrating.  Higher the frequency better sensitivity and better resolution  If the grain size is 1/10 the wavelength, the ultrasound will be significantly scattered.

Q7: When a material grain size is on the order of ______ wavelength or larger, excessive scattering of the ultrasonic beam affect test result. A. B. C. D.

1 ½ 1/10 1/100

2.5.3

Further Reading

Detectability variable:      

pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter (focal length → Cross sectional area), and the receiver circuitry of the instrument.

Investigating on: Sonic pulse volume ∝ pulse length, transducer Φ

Pulse Length: A sound pulse traveling through a metal occupies a physical volume. This volume changes with depth, being smallest in the focal zone. The pulse volume, a product of a pulse length L and a cross-sectional area A, can be fairly easily measured by combining ultrasonic A-scans and C-scans, as will be seen shortly. For many cases of practical interest, the inspection simulation models predict that S/N (signal to noise ratio) is inversely proportional to the square root of the pulse volume at the depth of the defect. This is known as the “pulse volume rule-of-thumb” and has become a guiding principle for designing inspections. Generally speaking, it applies when both the grain size and the lateral size of the defect are smaller than the sound pulse diameter. http://www.cnde.iastate.edu/ultrasonics/grain-noise

Determining cross sectional area using reflector- A Scan (6db drop)

Determining cross sectional area using reflector- C Scan

“Sonic pulse volume” and S/N (defect resolution)

Pulse volume rule-of-thumb: Competing grain noise ∝√(pulse volume)

2.6: Attenuation of Sound Waves 2.6.1 Material Attenuation: Attenuation by definition is the rate of decrease of sound energy when a ultrasound wave id propagating in a medium. The sound attenuation in material depends on heat treatment, grain size, viscous friction, crystal stricture (anisotropy or isotropy), porosity, elastic hysteresis, hardness, Young’s modulus, etc. Sound attenuations are affected by; (1) Geometric beam spread, (2) Absorption, (3) Scattering. Material attenuation affects item (2) & (3).

When sound travels through a medium, its intensity diminishes with distance. In idealized materials, sound pressure (signal amplitude) is only reduced by the (1) spreading of the wave. Natural materials, however, all produce an effect which further weakens the sound. This further weakening results from (2) scattering and (3) absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation. Absorption is the conversion of the sound energy to other forms of energy. The combined effect of scattering and absorption (spreading?) is called attenuation. Ultrasonic attenuation is the decay rate of the wave as it propagates through material. Attenuation of sound within a material itself is often not of intrinsic interest. However, natural properties and loading conditions can be related to attenuation. Attenuation often serves as a measurement tool that leads to the formation of theories to explain physical or chemical phenomenon that decreases the ultrasonic intensity.

Absorption: Sound attenuations are affected by; (1) Geometric beam spread, (2) Absorption, (3) Scattering. Absorption processes 1. Mechanical hysteresis 2. Internal friction 3. Others (?) For relatively non-elastic material, these soft and pliable material include lead, plastid, rubbers and non-rigid coupling materials; much of the energy is loss as heat during sound propagation and absorption is the main reason that the testing of these material are limit to relatively thin section/

Scattering: Grain Size and Wave Frequency The relative impact of scattering source of a material depends upon their grain sizes in comparison with the Ultrasonic sound wave length. As the scattering size approaches that of a wavelength, scattering by the grain is a concern. The effects from such scattering could be compensated with the use of increasing wavelength ultrasound at the cost of decreasing sensitivity and resolution to detection of discontinuities. Other effect are anisotropic columnar grain with different elastic behavior at different grain direction. In this case the internal incident wave front becomes distorted and often appear to change direction (propagate better in certain preferred direction) in respond to material anisotropy.

Anisotropic Columnar Grains with different elastic behavior at different grain direction.

Spreading/ Scattering / adsorption (reflection is a form of scattering) Adsorption

Scattering

Spreading

Scatterbrain

The amplitude change of a decaying plane wave can be expressed as:

In this expression Ao is the unattenuated amplitude of the propagating wave at some location. The amplitude A is the reduced amplitude after the wave has traveled a distance z from that initial location. The quantity α is the attenuation coefficient of the wave traveling in the z-direction. The α dimensions of are nepers/length, where a neper is a dimensionless quantity. The term e is the exponential (or Napier's constant) which is equal to approximately 2.71828.

The units of the attenuation value in Nepers per meter (Np/m) can be converted to decibels/length by dividing by 0.1151. Decibels is a more common unit when relating the amplitudes of two signals.

Attenuation is generally proportional to the square of sound frequency. Quoted values of attenuation are often given for a single frequency, or an attenuation value averaged over many frequencies may be given. Also, the actual value of the attenuation coefficient for a given material is highly dependent on the way in which the material was manufactured. Thus, quoted values of attenuation only give a rough indication of the attenuation and should not be automatically trusted. Generally, a reliable value of attenuation can only be obtained by determining the attenuation experimentally for the particular material being used.

Attenuation ∝ Frequency (f )2

Attenuation can be determined by evaluating the multiple back wall reflections seen in a typical A-scan display like the one shown in the image at the bottom. The number of decibels between two adjacent signals is measured and this value is divided by the time interval between them. This calculation produces a attenuation coefficient in decibels per unit time Ut. This value can be converted to nepers/length by the following equation.

Where v is the velocity of sound in meters per second and Ut is in decibels per second.

Amplitude at distance Z

where: Where v is the velocity of sound in meters per second and Ut is in decibels per second.

Ao

Ut A

2.6.2

Factors Affecting Attenuation:

1. Testing Factors  Testing frequency  Boundary conditions  Wave form geometry 2. Base Material Factors      

Material type Chemistry Integral constituents (fiber, voids, water content, inclusion, anisotropy) Forms (casting, wrought) Heat treatment history Mechanical processes(Hot or cold working; forging, rolling, extruding, TMCP, directional working)

2.6.3

Frequency selection

There is no ideal frequency; therefore, frequency selection must be made with consideration of several factors. Frequency determines the wavelength of the sound energy traveling through the material. Low frequency has longer wavelengths and will penetrate deeper than higher frequencies. To penetrate a thick piece, low frequencies should be used. Another factor is the size of the grain structure in the material. High frequencies with shorter wavelengths tend to reflect off grain boundaries and become lost or result in ultrasonic noise that can mask flaw signals. Low frequencies must be used with coarse grain structures. However, test resolution decreases when frequency is decreased. Small defects detectable at high frequencies may be missed at lower frequencies. In addition, variations in instrument characteristics and settings as well as material properties and coupling conditions play a major role in system performance. It is critical that approved testing procedures be followed.

2.6.4

Further Reading on Attenuation

Q94: In general, which of the following mode of vibration would have the greatest penetrating power in a coarse grain material if the frequency of the wave are the same? a) b) c) d)

Longitudinal wave Shear wave Transverse wave All the above modes would have the same penetrating power

Q: The random distribution of crystallographic direction in alloys with large crystalline structures is a factor in determining: A. B. C. D.

Acoustic noise levels Selection of test frequency Scattering of sound All of the above

Q168: Heat conduction, viscous friction, elastic hysteresis, and scattering are four different mechanism which lead to: A. B. C. D.

Attenuation Refraction Beam spread Saturation

Q7: When the material grain size is in the order of ____ wavelength or larger, excessive scattering of the ultrasound beam may affect test result: A. B. C. D.

1 ½ 1/10 1/100

2.7: Acoustic Impedance Acoustic impedance is a measured of resistance of sound propagation through a part.

From the table air has lower acoustic impedance than steel and for a given energy Aluminum would travel a longer distance than steel before the same amount of energy is attenuated.

Transmission & Reflection Animation: http://upload.wikimedia.org/wikipedia/commons/3/30/Partial_transmittance.gif

Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid. The acoustic impedance (Z) of a material is defined as the product of its density (p) and acoustic velocity (V).

Z = pV Acoustic impedance is important in: 1. the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances. 2. the design of ultrasonic transducers. 3. assessing absorption of sound in a medium.

The following applet can be used to calculate the acoustic impedance for any material, so long as its density (p) and acoustic velocity (V) are known. The applet also shows how a change in the impedance affects the amount of acoustic energy that is reflected and transmitted. The values of the reflected and transmitted energy are the fractional amounts of the total energy incident on the interface. Note that the fractional amount of transmitted sound energy plus the fractional amount of reflected sound energy equals one. The calculation used to arrive at these values will be discussed on the next page.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/applet_2_6/applet_2_6.htm

Reflection/Transmission Energy as a function of Z

Reflection/Transmission Energy as a function of Z

Q2.8: The acoustic impedance of material used to determined: A. Angle of refraction at the interface B. Attenuation of material C. Relative amount of sound energy coupled through and reflected at an interface D. Beam spread within the material

2.8: Reflection and Transmission Coefficients (Pressure) Ultrasonic waves are reflected at boundaries where there is a difference in acoustic impedances (Z) of the materials on each side of the boundary. (See preceding page for more information on acoustic impedance.) This difference in Z is commonly referred to as the impedance mismatch. The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface or boundary between one medium and another. The fraction of the incident wave intensity that is reflected can be derived because particle velocity and local particle pressures must be continuous across the boundary.

When the acoustic impedances of the materials on both sides of the boundary are known, the fraction of the incident wave intensity that is reflected can be calculated with the equation below. The value produced is known as the reflection coefficient. Multiplying the reflection coefficient by 100 yields the amount of energy reflected as a percentage of the original energy.

Since the amount of reflected energy plus the transmitted energy must equal the total amount of incident energy, the transmission coefficient is calculated by simply subtracting the reflection coefficient from one. Formulations for acoustic reflection and transmission coefficients (pressure) are shown in the interactive applet below. Different materials may be selected or the material velocity and density may be altered to change the acoustic impedance of one or both materials. The red arrow represents reflected sound and the blue arrow represents transmitted sound.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/applet_2_7/applet_2_7.htm

Reflection Coefficient:

Note that the reflection and transmission coefficients are often expressed in decibels (dB) to allow for large changes in signal strength to be more easily compared. To convert the intensity or power of the wave to dB units, take the log of the reflection or transmission coefficient and multiply this value times 10. However, 20 is the multiplier used in the applet since the power of sound is not measured directly in ultrasonic testing. The transducers produce a voltage that is approximately proportionally to the sound pressure. The power carried by a traveling wave is proportional to the square of the pressure amplitude. Therefore, to estimate the signal amplitude change, the log of the reflection or transmission coefficient is multiplied by 20.

Using the above applet, note that the energy reflected at a water-stainless steel interface is 0.88 or 88%. The amount of energy transmitted into the second material is 0.12 or 12%. The amount of reflection and transmission energy in dB terms are -1.1 dB and -18.2 dB respectively. The negative sign indicates that individually, the amount of reflected and transmitted energy is smaller than the incident energy.

If reflection and transmission at interfaces is followed through the component, only a small percentage of the original energy makes it back to the transducer, even when loss by attenuation is ignored. For example, consider an immersion inspection of a steel block. The sound energy leaves the transducer, travels through the water, encounters the front surface of the steel, encounters the back surface of the steel and reflects back through the front surface on its way back to the transducer. At the water steel interface (front surface), 12% of the energy is transmitted. At the back surface, 88% of the 12% that made it through the front surface is reflected. This is 10.6% of the intensity of the initial incident wave. As the wave exits the part back through the front surface, only 12% of 10.6 or 1.3% of the original energy is transmitted back to the transducer.

Incident Wave other than Normal? – Oblique Incident

http://www.slideshare.net/crisevelise/fundamentals-ofultrasound?related=1&utm_campaign=related&utm_medium=1&utm_sourc e=29

Incident Wave other than Normal? – Oblique Incident

Q: The figure above shown the partition of incident and reflected wave at water-Aluminum interface at an incident angle of 20, the reflected and transmitted wave are: A. B. C. D.

60% and 40% 40% and 60% 1/3 and 2/3 80% and 20%

Note: if normal incident the reflected 70% Transmitted 30%

Further Reading (Olympus Technical Note) The boundary between two materials of different acoustic impedances is called an acoustic interface. When sound strikes an acoustic interface at normal incidence, some amount of sound energy is reflected and some amount is transmitted across the boundary. The dB loss of energy on transmitting a signal from medium 1 into medium 2 is given by: dB loss of transmission = 10 log10 [ 4Z1Z2 / (Z1+Z2)2] The dB loss of energy of the echo signal in medium 1 reflecting from an interface boundary with medium 2 is given by: dB loss of Reflection = 10 log10 [ (Z1-Z2)2 / (Z1+Z2)2]

For example: The dB loss on transmitting from water (Z = 1.48) into 1020 steel (Z = 45.41) is -9.13 dB; this also is the loss transmitting from 1020 steel into water. The dB loss of the backwall echo in 1020 steel in water is -0.57 dB; this also is the dB loss of the echo off 1020 steel in water. The waveform of the echo is inverted when Z2 HB). The two signals may occur in either screen order and do not have to be successive if part of a multipleecho pattern. Unless otherwise specified in the requesting document, any test block that will produce such signals at the nominal test settings specified can be used. For many commonly used search units and test conditions, the test block shown in Fig. 1 will usually be satisfactory when the beam is directed along the H dimension toward the two holes. The method is applicable to either contact or immersion tests; however, if a choice exists, the latter may be preferable for ease of set-up and coupling stability……(more…)

4.2.9: Time Correction Gain (TCG) Please read: http://aqualified.com/tcg-dac-ndt-ultrasound/

Q61: The vertical linear range of a test instrument may be determined by obtaining ultrasonic responses from: A. B. C. D.

a set of distance amplitude blocks steel ball located at several different water path distances a set of area amplitude blocks all of the above

Q29: Test sensitivity correction for a metal distance and discontinuity area responses are accomplished by using: A. B. C. D.

An area amplitude set of blocks An area amplitude and a distance amplitude set of blocks A distance amplitude set of blocks Steel balls of varying diameters.

4.3: Curvature Correction Curvature in the surface of a component will have an effect on the shape of the ultrasonic beam. The image to the right shows the beam from a focused immersion probe being projected on to the surface of a component. Lighter colors represent areas of greater beam intensity. It can be seen that concave surfaces work to focus the beam and convex surfaces work to defocus the beam. Similar effects are also seen with contact transducers. When using the amplitude of the ultrasonic signal to size flaws or for another purpose, it is necessary to correct for surface curvature when it is encountered. The "correction" value is the change in amplitude needed to bring signals from a curved surface measurement to the flat surface or DAC value.

Convex surfaces work to defocus the beam

Diverge if the surface is convex.

Concave surface contourFocusing effects

Concave surfaces work to focus the beam

Diverge if the surface is convex.

Concave surface contourFocusing effects

convex surfaces work to defocus the beam Convex surfaces work to defocus the beam

Convex surfaces work to defocus the beam- When sound travels from a liquid through a metal, it will converge if the surface is concave or diverge if the surface is convex.

Q: In an immersion method, the incident sound path enter the specimen interface with convex geometry, the sound path on entry into the specimen, the convex surface works to a) b) c) d)

De-focus the sound Focus the sound Has no effect on the focusing or de-focusing the sound Reflected totally all the incident sound.

Q: In transmitting sound energy into a part shown below in a immersion testing, the sound beam will be: a) b) c) d)

Diverge Converge Straight into Will not enter

A curvature correction curve can be generated experimentally in a manner similar to that used to generate a DAC curve, This simply requires a component with a representative reflector at various distances below the curved surface. Since any change in the radius will change the focus of the sound beam, it may be necessary to develop reference standards with a range of surface curvatures. However, computer modeling can also be used to generate a close approximation of the curvature correction value. Work by Ying and Baudry (ASME 62-WA175, 1962) and by Birchak and Serabian (Mat. Eval. 36(1), 1978) derived methods for determining "correction factors" to account for change in signal amplitude as a function of the radius of curvature of convex, cylindrical components. An alternative model for contact and immersion probe inspection was more recently by researchers at the Center for NDE at Iowa State University. This mathematical model further predicts transducer radiation patterns using the Gauss-Hermite model, which has been used extensively for simulation of immersion mode inspections.

The resulting model allows computationally efficient prediction of the full ultrasonic fields in the component for 1. 2. 3. 4.

any frequency, including broadband measurements. both circular and rectangular crystal shapes. general component surface curvature both normal and oblique incidence (e.g., angle beam wedges) transducers.

When coupled with analytical models for defect scattering amplitudes, the model can be used to predict actual flaw waveforms. The image shown above was generated with this model.

The plot to the right shows an example curvature correction curve and DAC curve. This curvature correction curve was generated for the application of detecting a #4 flat bottom hole under a curved surface as shown in the sketch and photograph. An immersion techniques was used generate a shear wave since the reflective surface of the target flaw was not parallel with the surface. The DAC curve drops monotonically since the water path ensures that the near field of the sound beam is always outside the part. The correction factor starts out negative because of the focusing effect of the curved surface. At greater depths, the correction factor is positive due to the increased beam spread beyond the focal zone caused by the surface curvature.

Curvature Corrections

A table of correction values and the DAC and curvature correction curves for different size radiuses can be found at the following link.

https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/CalibrationMeth/table/table.htm

Curvature Correction

Curvature Correction

4.4: Calibration References & Standards What are standards? Standards are documented agreements containing technical specifications or other precise criteria to be used consistently as rules, guidelines, or definitions of characteristics, in order to ensure that materials, products, processes, and services are fit for their purpose. For example, the format of the credit cards, phone cards, and "smart" cards that have become commonplace is derived from an ISO International Standard. Adhering to the standard, which defines such features as an optimal thickness (0.76 mm), means that the cards can be used worldwide.

An important source of practice codes, standards, and recommendations for NDT is given in the Annual Book of the American Society of Testing and Materials, ASTM. Volume 03.03, Nondestructive Testing is revised annually, covering acoustic emission, eddy current, leak testing, liquid penetrant, magnetic particle, radiography, thermography, and ultrasonics. There are many efforts on the part of the National Institute of Standards and Technology (NIST) and other standards organizations, both national and international, to work through technical issues and harmonize national and international standards.

Reference Reflectors: are used as a basis for establishing system performance and sensitivity.

Spherical reflectors are often used in immersion techniques for assessing sound fields. 1. Omni direction 2. Sphere directivity patterns reduce reflectance as compare with plane reflector 3. Sphere of any materials could be used, however steel balls are often preferred.

Reference Reflectors are used as a basis for establishing system performance and sensitivity.

4.5: Questions & Answers Exercises

Q80: The 50 mm diameter hole in an IIW block is used to: (a) Determine the beam index point (b) Check resolution (c) Calibrate angle beam distance (d) Check beam angle Q81: The 100 mm radius in an IIW block is used to: (a) Calibrate sensitivity level (b) Check resolution (c) Calibrate angle beam distance (d) Check beam angle

Q6: The Notches are frequently used for reference reflectors for: A. B. C. D.

Distance amplitude calibration for shear wave Area amplitude calibration Thickness calibration of plate Determine of near surface resolution

Q17: Notches provide good reference discontinuities when UT examination is conducted to primarily detect defects such as: A. B. C. D.

Porosity in rolled plate Inadequate penetration at the root of weld Weld porosity Internal inclusion

4.6: Video Time

http://v.pps.tv/play_315ARS.html

Birring NDT Series, UT of Welds Part 1 of 2 - CALIBRATION

https://www.youtube.com/embed/SRJktrHUlM4

Birring NDT Series, Ultrasonic Testing # 4, Angle Beam Shear Wave UT as per AWS D1.1

www.youtube.com/embed/vXcAI-Zci30

Section 5: Measurement Techniques

Content: Section 5: Measurement Techniques 5.1: Normal Beam Inspection 5.2: Angle Beams 5.3: Reflector Sizing 5.4: Automated Scanning 5.5: Precision Velocity Measurements 5.6: Attenuation Measurements 5.7: Spread Spectrum Ultrasonics 5.8: Signal Processing Techniques 5.9: Scanning Methods 5.10: Scanning Patterns 5.11: Pulse Repetition Rate and Penetration 5.12: Interferences & Non Relevant Indications 5.13: Entry Surface Variables 5.14: The Concept of Effective Distance 5.15: Exercises

Expert at works

5.1: Normal Beam Inspection Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the wellknown relationship:

where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip transit time. d = vt/2 or v = 2d/t

d1 = v½t

d2 = v½t

= d1+d2

2vt

2vt

A-Scan

A Scan

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/MeasurementTech/applet_4_1/applet_4_1.htm

Precision ultrasonic thickness gages usually operate at frequencies between 500 kHz and 100 MHz, by means of piezoelectric transducers that generate bursts of sound waves when excited by electrical pulses. A wide variety of transducers with various acoustic characteristics have been developed to meet the needs of industrial applications. Typically, 1. lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, 2. while higher frequencies will be recommended to optimize resolution in thinner, non-attenuating, non-scattering materials.

0.5 MHz ~ 100 MHz

In thickness gauging, ultrasonic techniques permit quick and reliable measurement of thickness without requiring access to both sides of a part. Accuracy's as high as ±1 micron or ±0.0001 inch can be achieved in some applications. It is possible to measure most engineering materials ultrasonically, including metals, plastic, ceramics, composites, epoxies, and glass as well as liquid levels and the thickness of certain biological specimens. On-line or in-process measurement of extruded plastics or rolled metal often is possible, as is measurements of single layers or coatings in multilayer materials. Modern handheld gages are simple to use and very reliable.

5.2: Angle Beams I Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. An angled sound path allows the sound beam to come in from the side, thereby improving detectability of flaws in and around welded areas.

Ɵ = Angle of reflection, T=Material thickness, S= Sound path, Surface distance = SinƟ x S, Depth= CosƟ x S

A-Scan

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/MeasurementTech/applet_4_2/applet_4_2.htm

Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. The geometry of the sample below allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas.

Ɵ = Angle of reflection, T=Material thickness, S= Sound path, Skip = 2(T x TanƟ), Leg = T/CosƟ, V Path = 2 x Leg

A-Scan

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/MeasurementTech/applet_4_3/applet_4_3.htm

Flaw Location and Echo Display

Flaw Location and Echo Display

Flaw Location and Echo Display

Flaw Location and Echo Display

Flaw Location and Echo Display

Flaw Location and Echo Display

Dead Zone

Near Surface Detectability with Angle Beam Transducer

Flaw Location

Flaw Location with Angle Beam Transducer

Flaw Location with Angle Beam Transducer

Flaw Location with Angle Beam Transducer

Flaw Location with Angle Beam Transducer

Why angle beam assemblies are used Cracks or other discontinuities perpendicular to the surface of a test piece, or tilted with respect to that surface, are usually invisible with straight beam test techniques because of their orientation with respect to the sound beam. Perpendicular cracks do not reflect any significant amount of sound energy from a straight beam because the beam is looking at a thin edge that is much smaller than the wavelength, and tilted cracks may not reflect any energy back in the direction of the transducer. This situation can occur in many types of welds, in structural metal parts, and in many other critical components. An angle beam assembly directs sound energy into the test piece at a selected angle. A perpendicular crack will reflect angled sound energy along a path that is commonly referred to as a corner trap, as seen in the illustration below.

/

http://www.olympus-ims.com/en/applications/angle-beam-transducers

The angled sound beam is highly sensitive to cracks perpendicular to the far surface of the test piece (first leg test) or, after bouncing off the far side, to cracks perpendicular to the coupling surface (second leg test). A variety of specific beam angles and probe positions are used to accommodate different part geometries and flaw types. In the case of angled discontinuities, a properly selected angle beam assembly can direct sound at a favorable angle for reflection back to the transducer.

/

http://www.olympus-ims.com/en/applications/angle-beam-transducers

How they work -- Snell's Law A sound beam that hits a surface at perpendicular incidence will reflect straight back. A sound beam that hits a surface at an angle will reflect forward at the same angle.

Sound energy that is transmitted from one material to another bends in accordance with Snell's Law of refraction. Refraction is the bending of a sound beam (or any other wave) when it passes through a boundary between two materials of different velocities. A beam that is traveling straight will continue in a straight direction, but a beam that strikes a boundary at an angle will be bent according to the formula:

Typical angle beam assemblies make use of mode conversion and Snell's Law to generate a shear wave at a selected angle (most commonly 30, 45, 60, or 70 degrees) in the test piece. As the angle of an incident longitudinal wave with respect to a surface increases, an increasing portion of the sound energy is converted to a shear wave in the second material, and if the angle is high enough, all of the energy in the second material will be in the form of shear waves.

There are two advantages to designing common angle beams to take advantage of this mode conversion phenomenon: (1) First, energy transfer is more efficient at the incident angles that generate shear waves in steel and similar materials. (2) Second, minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% the wavelength of a comparable longitudinal wave, and minimum flaw size resolution increases as the wavelength of a sound beam gets smaller.

Selecting the right angle beam assembly The parameters that affect angle beam performance include not only the (1) beam angle generated by the wedge, but also (2) transducer frequency and (3) element size. The optimum beam angle will generally be governed by the geometry of the test piece and the orientation of the discontinuities that the test is intended to find. Transducer frequency affects penetration and flaw resolution: 1. As frequency increases, the distance the sound wave will travel in a given material decreases, but resolution of small discontinuities improves. 2. As frequency decreases, the distance the sound wave will travel increases but the minimum detectable flaw size will become larger. 3. Similarly, larger element sizes may decrease inspection time by increasing coverage area, but the reflected echo amplitude from small discontinuities will decrease. Smaller element sizes will increase reflection amplitude from small discontinuities, but the inspection may take longer because the smaller beam covers less area. These conflicting factors must be balanced in any given application, based on specific test requirements.

Contoured wedges

The IIW recommends the use of a contoured wedge whenever the gap between the wedge and the test surface exceeds 0.5 mm (approximately 0.020 in.). Under this guideline, a contoured wedge should be used whenever part radius is less than the square of a wedge dimension (length or width) divided by four:

where R = radius of test surface W = width of wedge if testing in axial orientation, length of wedge if testing in circumferential orientation Of course switching to a small wedge, if possible within the parameters of inspection requirements, will improve coupling on curved surfaces. As a practical matter, contouring should be considered whenever signal strength diminishes or couplant noise increases to a point where the reliability of an inspection is impaired.

Focused dual element angle beams The vast majority of angle beam assemblies use single element, unfocused transducers. However, in some tests involving highly attenuating or scattering materials such as coarse grain cast stainless steel, focused dual element angle beams are useful. Because they have separate transmitting and receiving elements, dual element transducers can typically be driven at higher excitation energies without noise problems associated with ringdown or wedge noise. Focusing permits a higher concentration of sound energy at a selected depth within the test piece, increasing sensitivity to discontinuities in that region.

High temperature wedges Standard angle beam assemblies are designed for use at normal environmental temperatures only. For situations where metal must be inspeced at elevated temperature, special high temperature wedges are available. Some of these wedges will tolerate brief contact with surfaces as hot as 480° C or 900° F. However, it is important to note that high temperature wedges require special attention with regard to the sound path they generate. With any high temperature wedge, sound velocity in the wedge material will decrease as it heats up, and thus the refracted angle in metals will increase as the wedge heats up. If this is of concern in a given test, refracted angle should be verified at actual operating temperature. As a practical matter, thermal variations during testing will often make precise determination of the actual refracted angle difficult.

Surfaces as hot as 480°C / 900°F

threaded

snap-in

steel with a shear wave velocity of approximately 3,250 M/S or 0.1280 in/uS.

5.3: Reflector Sizing There are many sizing methods, these include: 5.3.1

Crack Tip Diffraction

When the geometry of the part is relatively uncomplicated and the orientation of a flaw is well known, the length (a) of a crack can be determined by a technique known as tip diffraction. One common application of the tip diffraction technique is to determine the length of a crack originating from on the backside of a flat plate as shown below. In this case, when an angle beam transducer is scanned over the area of the flaw, the principle echo comes from the base of the crack to locate the position of the flaw (Image 1). A second, much weaker echo comes from the tip of the crack and since the distance traveled by the ultrasound is less, the second signal appears earlier in time on the scope (Image 2).

Crack Tip Diffraction Methods

No animation.

Crack height (a) is a function of the ultrasound velocity (v) in the material, the incident angle (Q2) and the difference in arrival times between the two signal (dt). Since the incident angle and the thickness of the material is the same in both measurements, two similar right triangle are formed such that one can be overlayed on the other. A third similar right triangle is made, which is comprised on the crack, the length dt and the angle Q2. The variable dt is really the difference in time but can easily be converted to a distance by dividing the time in half (to get the one-way travel time) and multiplying this value by the velocity of the sound in the material. Using trigonometry an equation for estimating crack height from these variables can be derived as shown below.

Crack Tip Diffraction Method

The equation is complete once distance dt is calculated by dividing the difference in time between the two signals (dt) by two and multiplying this value by the sound velocity.

5.3.2 6 dB Drop SizingFor Large Reflector (greater than beam width), i.e. there is no BWE.

6 dB Drop Method

6 dB Drop Method

6 dB Drop Method

www.youtube.com/embed/hsR17WA3nHg

6 dB Drop Method

5.3.3

The 20 dB drop sizing method

We can use a beam plot to find the edge of a defect by using the edge of the sound beam. If we know the width of a beam at a certain distance from the crystal, we can mark the distance across a defect from where the extreme edges of the beam touch each end of the defect and then subtract the beam width to get the defect size. When the signal from the defect drops by 20dB from its peak, we judge that the edge of the beam is just touching the end of the defect. We can find the width of the sound beam at that range by consulting the beam plot that we have made Note: The peak of the defect is normally taken as being the last peak on the screen before the probe goes off the end of the defect, not necessarily the maximum signal from a defect.

20 dB Drop Method

20 dB Drop Sizing- For Small Reflector (smaller than beam width). To use this method the transducer beam width need to be first determined.

Construction of a beam edge plot -20dB – Normal Beam Find the hole at a depth of 13mm on an IOW block with a 0 degree probe and maximise the signal. Move the probe until you get the highest signal you can from the hole, then turn the signal to FSH using gain. Mark the position of the middle of the probe on the side of the block.

Move the probe to one side until the signal drops to 10%FSH (-20dB) and mark the centre of the probe on the side of the block.

Move the probe to the other side of the hole until the signal drops to 10%FSH (-20dB) and mark the centre of the probe on the block.

Use the distances between the marks on the block to plot the beam on a piece of graph paper. Measure 13mm depth on the paper then mark the distances of the probe centre at -20dB from the beam centre at 100%FSH on either side.

Now find the 25mm hole and maximise the signal, turning it to 100%FSH. Move the probe to either side of the hole marking the centre of the probe on the side of the block where the signal drops by 20dB. Measure 25mm on the paper and use the distances on the block to plot the beam dimensions at 25mm.

Repeat using the 32mm hole. Join up the points marking the probe centre at 20dB to obtain a beam plot.

Note that we have only drawn the beam width in one plane, so the probe must be marked accordingly and used to measure defects in this plane. We use knowledge of the beam spread to size defects, find the edges and hence their width, length and sometimes orientation.

Construction of a beam edge plot -20dB – Angle Beam

5.3.4 Equalization Back Wall Sizing- The probe moving off the edges of the reflector until the amplitude is equal to the rising BWE

5.3.5 Maximum Amplitude Techniques The technique is used for small reflector. The probe moving off the edges of the reflector until the amplitude is maximum and the line joining the boundary is the size of reflector cluster.

5.3.6 The DGS Method Distance Gain Size Method. The technique is used to find the equivalent reflector size by comparing the gain between the flaw and the known size reflector.

5.4: Automated Scanning Ultrasonic scanning systems are used for automated data acquisition and imaging. They typically integrate a ultrasonic instrumentation, a scanning bridge, and computer controls. The signal strength and/or the time-of-flight of the signal is measured for every point in the scan plan. The value of the data is plotted using colors or shades of gray to produce detailed images of the surface or internal features of a component. Systems are usually capable of displaying the data in A-, B- and C-scan modes simultaneously. With any ultrasonic scanning system there are two factors to consider: ■ how to generate and receive the ultrasound. ■ how to scan the transducer(s) with respect to the part being inspected.

Automatic Scanning

The most common ultrasonic scanning systems involve the use of an immersion tank as shown in the image above. The ultrasonic transducer and the part are placed under water so that consistent coupling is maintained by the water path as the transducer or part is moved within the tank. However, scanning systems come in a large variety of configurations to meet specific inspection needs. In the image to the right, an engineer aligns the heads of a squirter system that uses a through-transmission technique to inspect aircraft composite structures. In this system, the ultrasound travels through columns of forced water which are scanned about the part with a robotic system. A variation of the squirter system is the "Dripless Bubbler" scanning system, which is discussed below.

Dripless Bubbler

It is often desirable to eliminate the need for the water coupling and a number of state-of-the-art UT scanning systems have done this. Laser ultrasonic systems use laser beams to generate the ultrasound and collect the resulting signals in an noncontact mode. Advances in transducer technology has lead to the development of an inspection technique known as air-coupled ultrasonic inspection. These systems are capable of sending ultrasonic energy through air and getting enough energy into the part to have a useable signal. These system typically use a through-transmission technique since reflected energy from discontinuities are too weak to detect.

The second major consideration is how to scan the transducer(s) with respect to the part being inspected. When the sample being inspected has a flat surface, a simple raster-scan can be performed. If the sample is cylindrical, a turntable can be used to turn the sample while the transducer is held stationary or scanned in the axial direction of the cylinder. When the sample is irregular shaped, scanning becomes more difficult. As illustrated in the beam modeling animation, curved surface can steer, focus and defocus the ultrasonic beam. For inspection applications involving parts having complex curvatures, scanning systems capable of performing contour following are usually necessary.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/AppleScan/Apple2.swf

5.5: Precision Velocity Measurements Changes in ultrasonic wave propagation speed, along with energy losses, from interactions with a materials microstructures are often used to nondestructively gain information about a material's properties. Measurements of sound velocity and ultrasonic wave attenuation can be related to the elastic properties that can be used to characterize the texture of polycrystalline metals. These measurements enable industry to replace destructive microscopic inspections with nondestructive methods. Of interest in velocity measurements are longitudinal wave, which propagate in gases, liquids, and solids. In solids, also of interest are transverse (shear) waves. The longitudinal velocity is independent of sample geometry when the dimensions at right angles to the beam are large compared to the beam area and wavelength. The transverse velocity is affected little by the physical dimensions of the sample.

Pulse-Echo and Pulse-Echo-Overlap Methods Rough ultrasonic velocity measurements are as simple as measuring the time it takes for a pulse of ultrasound to travel from one transducer to another (pitch-catch) or return to the same transducer (pulse-echo). Another method is to compare the phase of the detected sound wave with a reference signal: slight changes in the transducer separation are seen as slight phase changes, from which the sound velocity can be calculated. These methods are suitable for estimating acoustic velocity to about 1 part in 100. Standard practice for measuring velocity in materials is detailed in ASTM E494.

ASTM E494 - 10 Measuring Ultrasonic Velocity in Materials Active Standard ASTM E494 | Developed by Subcommittee: E07.06 Book of Standards Volume: 03.03

Precision Velocity Measurements (using EMATs) Electromagnetic-acoustic transducers (EMAT) generate ultrasound in the material being investigated. When a wire or coil is placed near to the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region. If a static magnetic field is also present, these currents will experience Lorentz forces of the form F=JxB where F is a body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction.

EMATs

http://www.resonic.com/error%20scan.swf http://www.resonic.com/scan2.swf

http://www.resonic.com/emar_how_it_works.html

The most important application of EMATs has been in nondestructive evaluation (NDE) applications such as flaw detection or material property characterization. Couplant free transduction allows operation without contact at elevated temperatures and in remote locations. The coil and magnet structure can also be designed to excite complex wave patterns and polarizations that would be difficult to realize with fluid coupled piezoelectric probes. In the inference of material properties from precise velocity or attenuation measurements, use of EMATs can eliminate errors associated with couplant variation, particularly in contact measurements. Differential velocity is measured using a T1-T2---R fixed array of EMAT transducer at 0, 45°, 90° or 0°, 90° relative rotational directions depending on device configuration:

EMAT Driver Frequency: 450-600 KHz (nominal) Sampling Period: 100 ns Time Measurement Accuracy: -- Resolution 0.1 ns -- Accuracy required for less than 2 KSI Stress Measurements: Variance 2.47 ns -- Accuracy required for texture: Variance 10.0 Ns ------ W440 < 3.72E-5 ------ W420 < 1.47E-4 ------ W400 < 2.38E-4

Time Measurement Technique Fourier Transform-Phase-Slope determination of delta time between received RF bursts (T2-R) - (T1-R), where T2 and T1 EMATs are driven in series to eliminate differential phase shift due to probe liftoff.

Slope of the phase is determined by linear regression of weighted data points within the signal bandwidth and a weighted y-intercept. The accuracy obtained with this method can exceed one part in one hundred thousand (1:100,000).

5.6: Attenuation Measurements Ultrasonic wave propagation is influenced by the microstructure of the material through which it propagates. The velocity of the ultrasonic waves is influenced by the elastic moduli and the density of the material, which in turn are mainly governed by the amount of various phases present and the damage in the material. Ultrasonic attenuation, which is the sum of the (1)absorption and the (2)scattering, is mainly dependent upon the damping capacity and scattering from the grain boundary in the material. However, to fully characterize the attenuation required knowledge of a large number of thermo-physical parameters that in practice are hard to quantify. Ao Ut A

Relative measurements such as the change of attenuation and simple qualitative tests are easier to make than absolute measure. Relative attenuation measurements can be made by examining the exponential decay of multiple back surface reflections. However, significant variations in microstructural characteristics and mechanical properties often produce only a relatively small change in wave velocity and attenuation. Absolute measurements of attenuation are very difficult to obtain because the echo amplitude depends on factors in addition to amplitude.

The most common method used to get quantitative results is to use an ultrasonic source and detector transducer separated by a known distance. By varying the separation distance, the attenuation can be measured from the changes in the amplitude. To get accurate results, the influence of coupling conditions must be carefully addressed. To overcome the problems related to conventional ultrasonic attenuation measurements, ultrasonic spectral parameters for frequency-dependent attenuation measurements, which are independent from coupling conditions are also used. For example, the ratio of the amplitudes of higher frequency peak to the lower frequency peak, has been used for microstructural characterization of some materials.

Attenuation:

Ao

Ut A

Attenuation:

5.7: Spread Spectrum Ultrasonics Spread spectrum ultrasonics makes use of the correlation of continuous signals rather than pulse-echo or pitch-catch techniques. Spread spectrum ultrasonics is a patented new broad band spread-spectrum ultrasonic nondestructive evaluation method. In conventional ultrasonics, a pulse or tone burst is transmitted, then received echoes or throughtransmission signals are received and analyzed. In spread spectrum ultrasonics, encoded sound is continuously transmitted into the part or structure being tested. Instead of receiving echoes, spread spectrum ultrasonics generates an acoustic correlation signature having a one-to-one correspondence with the acoustic state of the part or structure (in its environment) at the instant of the measurement. In its simplest embodiment, the acoustic correlation signature is generated by cross correlating an encoding sequence, with suitable cross and auto correlation properties, transmitted into a part (structure) with received signals returning from the part (structure).

Section of bi-phase modulated spread spectrum ultrasonic waveform

Multiple probes may be used to ensure that acoustic energy is propagated through all critical volumes of the structure. Triangulation may be incorporated with multiple probes to locate regions of detected distress. Spread spectrum ultrasonics can achieve very high sensitivity to acoustic propagation changes with a low level of energy.

Two significant applications of Spread Spectrum Ultrasonics are: 1. Large Structures that allow ultrasonic transducers to be "permanently" affixed to the structures, eliminating variations in transducer registration and couplant. Comparisons with subsequent acoustic correlation signatures can be used to monitor critical structures such as fracture critical bridge girders. In environments where structures experience a great many variables such as temperature, load, vibration, or environmental coupling, it is necessary to filter out these effects to obtain the correct measurements of defects. In the example below, simulated defects were created by setting a couple of steel blocks on the top of the bridge girder.

Spread Spectrum UT

2. Piece-part assembly line environments where transducers and couplant may be precisely controlled, eliminating significant variations in transducer registration and couplant. Acoustic correlation signatures may be statistically compared to an ensemble of known "good" parts for sorting or accepting/rejecting criteria in a piece-part assembly line environment. Impurities in the incoming steel used to forge piece parts may result in sulfite stringer inclusions. In this next example simulated defects were created by placing a magnetized steel wire on the surface of a small steel cylindrical piston used in hydraulic transmissions.

Two discrimination technique are tested here, which are SUF-1 and SUF-2, with the latter giving the best discrimination between defect conditions. The important point being that spread spectrum ultrasonics can be extremely sensitive to the acoustic state of a part or structure being tested, and therefore, is a good ultrasonic candidate for testing and monitoring, especially where scanning is economic unfeasible.

EMATs with Spread Spectrum Ultrasonic

http://www.resonic.com/error%20scan.swf http://www.resonic.com/scan2.swf

http://www.resonic.com/emar_how_it_works.html

5.8: Signal Processing Techniques Signal processing involves techniques that improve our understanding of information contained in received ultrasonic data. Normally, when a signal is measured with an oscilloscope, it is viewed in the time domain (vertical axis is amplitude or voltage and the horizontal axis is time). For many signals, this is the most logical and intuitive直观的 way to view them. Simple signal processing often involves the use of gates to isolate the signal of interest or frequency filters to smooth or reject unwanted frequencies. When the frequency content of the signal is of interest, it makes sense to view the signal graph in the frequency domain. In the frequency domain, the vertical axis is still voltage but the horizontal axis is frequency.

Display

Time/Magnitude domain

Frequency /Magnitude domain

The frequency domain display shows how much of the signal's energy is present as a function of frequency. For a simple signal such as a sine wave, the frequency domain representation does not usually show us much additional information. However, with more complex signals, such as the response of a broad bandwidth transducer, the frequency domain gives a more useful view of the signal. Fourier theory says that any complex periodic waveform can be decomposed into a set of sinusoids with different amplitudes, frequencies and phases. The process of doing this is called Fourier Analysis, and the result is a set of amplitudes, phases, and frequencies for each of the sinusoids that makes up the complex waveform. Adding these sinusoids together again will reproduce exactly the original waveform. A plot of the frequency or phase of a sinusoid against amplitude is called a spectrum.

Fourier Analysis

Fourier Analysis

Fourier Analysis

The following Fourier Java applet, adapted with permission of Stanford University, allows the user to manipulate discrete time domain or frequency domain components and see the relationships between signals in time and frequency domains. The top row (light blue color) represents the real and imaginary parts of the time domain. Normally the imaginary part of the time domain signal is identically zero. The middle row (peach color) represents the the real and imaginary parts of the frequency domain. The bottom row (light green color) represents the magnitude (amplitude) and phase of the frequency domain signal. Magnitude is the square root of the sum of the squares of the real and imaginary components. Phase is the angular relationship of the real and imaginary components. Ultrasonic transducer manufactures often provide plots of both time domain and frequency domain (magnitude) signals characteristic of each transducer. Use this applet to explore the relationship between time and frequency domains.

Fourier Analysis

5.9: Scanning Methods Direct contact,

Direct contact,

single element probe

dual element probe

Through transmission

Fixed delay

Immersion testing

5.9.1

Pulse Echo Method

Pulse Echo Method: Sound pressure on axis (schematic) for the incident wave (top) and the wave reflected from a reflector in form a circular disc (bottom).

Pulse Echo Method

Pulse Echo Method

Pulse Echo Method- Schematic screen pictures obtained by the pulse-echo method. a Small flaw in sound beam; b two small flaws in sound beam; c large flaw in sound beam, smaller second flaw and back wall masked; d large, obliquely orientated flaw, back wall masked; e small flaw but no back wall echo because the axis of the beam is not incident at right angles on back wall; f strong attenuation of sound beam due to scattering, no echo from flaw or back wall, only "grass"

Pulse Echo Method

Pulse Echo Method- Multiple echoes in a plate. a schematic; b actual screen picture without time or thickness scale; steel plate 50 mm thick, frequency 4 MHz

Amplitude loss: Inverse Square Law

Influence of Shadow on axial defects

Influence of reflector orientation on signal

Influence of reflector size on signal

Pulse Echo Method

IP BE

F

plate

delamination

0

2

4

6

8

IP = Initial pulse F = Flaw BE = Backwall echo

10

Pulse Echo Method

s

Probe

Sound travel path

Work piece

Flaw

5.9.2

Pitch-Catch Methods

Advantage: Sensitive to near surface defect Capable of penetrating thicker material due to pitch-catch mode. Disadvantage: It measures only sound energy loss at the receiver, without giving details information of location.

5.9.2.1 Pitch-Catch Methods- Through Transmission Through transmission testing uses two search units; one unit is used as a transmitter and the other unit is used as a receiver, as shown in Figure below. With this technique, the ultrasonic beam passes through the test piece or is attenuated by one or more discontinuities. Total or partial attenuation of the signal is possible depending on the severity of the discontinuity. Both transducers must be properly coupled with a liquid coupling agent to obtain reliable results. As with other techniques using two search units, greater efficiency may be obtained by using a ceramic element in the transmitting search unit and a lithium sulfate element in the receiving unit.

Pitch-Catch Methods- Through Transmission

Pitch-Catch Methods- Through Transmission

Through transmission signal

1

T

R

1

2

T

R

2 0

Flaw

2

4

6

Back wall Echo

8

10

5.9.2.2 Pitch-Catch Methods- Tandem The tandem method, the examination is normally carried out using two similar 45° angle probes, one probe operating as the transmitter and the other probe as receiver. For wall thicknesses greater than approximately 160 mm, probes with different transducer sizes are preferred in order to ensure approximately the same beam diameters in the examination zone. The use of probe angles other than 45° may be necessary to comply with particular geometrical conditions. Probe angles that give rise to mode conversions shall be avoided. The probes are located in a line with their acoustic axis in the same direction. In this way the sound beam from the rear probe will, after reflection from the opposite surface, intersect the sound beam from the front probe at the centre of the examination zone.

Extract from: EN 583-4 Non-destructive testing - Ultrasonic examination - Part 4: Examination for discontinuities perpendicular to the surface

Figure 1 shows the relationship between the spacing of the probes (y) and the examination depth of the cross point (tm) and the height of the examination zone (tz). When examining objects with plane parallel surfaces the distance between the probes can be defined using the following equation: y = 2 tan α (d – tm) or 2 tan α (bottom depth)

Distance Between Transmitter / Receiver Probes y Depth tm

α x

α Plate Thickness d

L tan α = L / (d + d - tm) , L = 2d- tm tan α , tan α = x / tm , x = tm tan α y = L - x = (2d - tm tan α) – (tm tan α) y = tan α (2d-tm- tm) = 2 tan α (d - tm)

Video on Through Transmission Methods

www.youtube.com/embed/bRgCLb2cDU4?list=UUSOUDD4-FPV4tzqvUnquwXQ

5.9.3

Immersion Methods

Many of the same techniques used in contact testing can be used in immersion testing. One advantage of immersion testing is that water makes a very effective coupling agent. A wetting agent is often used with the water to reduce surface tension and minimize air bubble formation on probes and test parts. The main advantages of immersion testing are: ■ ■ ■ ■

speed of inspection, immersion medium provides excellent coupling, ability to direct the sound at any desired angle, and the ease of incorporating automatic scanning techniques.

With immersion testing, the time to send the beam through the water is usually greater than the time to send the beam through the test piece. All immersion search units are basically straight beam units that are directed to produce either longitudinal or shear waves in the test material.

Immersion Methods For immersion testing of steel and aluminum in water, the water path shall be at least 1” for every 4” thickness of the specimen (or ¼ of specimen thickness minimum). If the transducer is too close, the 2nd front reflection will appeared between the 1st front reflection and the 1st backwall echo and this may be wrong interpreted as discontinuity.

Immersion Methods- Since sound waves travel about four times faster in steel and aluminum than they do in water, a general rule of thumb is that the water distance should be 1/4 Ts the part thickness plus 1/4 in (6mm). When immersion testing is used for tapered plates, there should be a uniform water path above the test surface. With immersion testing, false indications from contoured surfaces will result in broad-based noise echoes.

Ts Minimum + [¼” 6mm (?)]

Immersion Methods- The water path shall be ¼ of specimen thickness minimum. (plus 6mm)

Minimum + [¼ “(?)]

Modified Immersion Methods- Bubbler Chamber

Modified Immersion Methods – Irrigation Dam

Angle Beam Immersion Methods Note the small front surface reflection. This due to the inclined incident angle reflected away from the transducer.

Straight Beam Immersion Methods

1

2 water delay

surface = sound entry backwall

flaw

IP

1

IE

IP

2

IE

BE

BE F

0

2

4

6

8

10

0

2

4

6

8

10

Angle Beam Immersion Methods- Pipe & Tubing Testing .

Angle Beam Immersion Methods- Weld Testing

Immersion Testing Set-up

Immersion Testing Set-up

Manipulators The manipulator is primarily intended to provide a means of scanning the test specimen with an immersed transducer.  The manipulator is mounted on a traversing mechanism, which allows movement of the manipulator from side to side.  The traversing mechanism is an integral component of the bridge assembly. A search tube is usually held rigid at right angles to the surface of the test specimen. Locking knobs are provided on the manipulator to allow positioning of the search tube in two planes for angle-beam testing.

Manipulators Bridge

Bridge Manipulator

Bridges When the manipulator is automated, electric motors are added to power the bridge carriage, the traversing mechanism, and the up and down movement of the search tube. The pulse-echo unit and the recording unit are also mounted on the bridge, with all power cords secured overhead to allow movement of the bridge along the full length of the tank.

Wands / Support Tubes The support tube for the immersion probe is sometimes called a wand. Its vertical height can be adjusted to vary water path distance and the adjuster which can manipulate probe angle of incidence at the tip of the wand.

Immersion Testing Set-up

Manipulator

Bridge

Wand / Tube

Immersion Testing Set-up

Manipulator

Bridge

Wand / Tube

Immersion Testing Set-up Manipulator

Bridge Wand / Tube

Other Reading (Olympus)- Angle Beam Immersion Methods Immersion transducers offer three major advantages over contact transducers: 1. Uniform coupling reduces sensitivity variations. 2. Reduction in scan time due to automated scanning. 3. Focusing of immersion transducers increases sensitivity to small reflectors. Focusing Configurations Immersion transducers are available in three different configurations: • unfocused (“flat”), • spherically (“spot”) focused, and • cylindrically (“line”) focused. Focusing is accomplished by either the addition of a lens or by curving the element itself. The addition of a lens is the most common way to focus a transducer.

An unfocused transducer may be used in general applications or for penetration of thick materials. A spherically focused transducer is commonly used to improve sensitivity to small flaws and a cylindrical focus is typically used in the inspection of tubing or bar stock. Examples of spherical and cylindrical focusing are shown in Figure (17) below.

Cylindrical

Spherical

Unfocused transducer By definition, the focal length of a transducer is the distance from the face of the transducer to the point in the sound field where the signal with the maximum amplitude is located. In an unfocused transducer, this occurs at a distance from the face of the transducer which is approximately equivalent to the transducer’s near field length. Because the last signal maximum occurs at a distance equivalent to the near field, a transducer, by definition, can not be acoustically focused at a distance greater than its near field.

Focus may be designated in three ways: FPF (Flat Plate Focus) - For an FPF focus, the lens is designed to produce a maximum pulse/echo response from a flat plate target at the distance indicated by the focal length PTF (Point Target Focus) - For a PTF focus, the lens is designed to produce a maximum pulse/echo response from a small ball target at the distance indicated by the focal length OLF (Optical Limit Focus) - The OLF designation indicates that the lens is designed according to the lens maker’s formula from physical optics and without reference to any operational definition of focal length. The OLF designation describes the lens and ignores diffraction effects.

Video on Immersion Testing

www.youtube.com/embed/W07-Z9at=UUSOUDD4-FPV4tzqvUnquwXQ

Q: In immersion testing, to remove the second water reflection (2nd entry surface signal) from between the entry surface signal and the first back reflection, you should: A. B. C. D.

Increase repetition rate Decrease frequency Decrease sweep length Increase water depth

Q110: In addition to other functions, a transducer manipulator in a mechanical immersion-scanning unit permits: A. B. C. D.

Use of the through transmission techniques Use of high scanning speed Detection of obliquely oriented discontinuities Utilization of skill operators

Q1: Which of the following scanning methods could be classified as an immersion type test? A. Tank in which the transducer and test piece are immersed B. Squirter bubbler method in which the sound is transmitted in a column of flowing water C. Scanning with a wheel-type transducer with the transducer inside a liquid filled tire D. All of the above

Q2: In an immersion test of a piece of steel or aluminum, the water distance appears on the display as a fairly wide space between the initial pulse and the front surface reflection because of: A. Reduced velocity of sound in water as compared to test specimen B. Increased velocity of sound in water as compared to test specimen C. Temperature of the water D. All of the above

Q2: Using the immersion method, a distance amplitude curve (DAC) for a 19 mm diameter, 5 MHz transducer shows the high point of the DAC at the B/51 mm block. One day later, the high point of the DAC for the same transducer is at the J/102 mm block. Assuming the calibration has not change, this would indicate that the transducer: A. B. C. D.

Is improving in resolution Is becoming defective Has the beam of smaller transducer Both A & B

Hint: B leads to C, thus D is the standard answer. http://www.ndt-instrument.com/UltrasonicThicknessGauge.asp?sort=Ultrasonic+Flaw+Detector

Q176: To evaluate and accurately locate discontinuities after scanning a part with a paintbrush transducer, it is generally necessary to use a: A. Transducer with a smaller crystal B. Scrubber C. Grid map D. Crystal collimator

38. The component in a conventional immersion system which spans the width of the immersion tank is called: A. An articulator. B. A bridge. C. A manipulator. D. A search tube.

5.10: Scanning Patterns

Scanning Patterns

5.11: Pulse Repetition Rate and Penetration The energy of the generated sound depend on the pulse repetition rate, the higher the repetition rate the higher the energy and the sound able to penetrate thicker material. However if the PRR is excessive, ghost signal may formed, this is due to the fact that the next sequence of pulse is generated before the expected returning signal reaching the receiver. 1. The pulse repetition frequency or pulse repetition rate PRR: is the number of pulse of ultrasonic energy that leave the probe in a given time (per second). Each pulse of energy that leave the probe must return before the next pulse leave, otherwise they will collide causing ghost echoes. 2. Transit time: The time taken for the pulse to travel from the probe and return 3. Clock interval: The time between pulse leaving the probe. The transit time must be shorter than the Clock interval else, ghost signal may formed. Typically the Clock interval should be 5 time the transit time.

PRR- Pulse Repetitive Frequency/Rate and Maximum Testable Thickness Clock interval = 1/PRR When Transit time = Clock interval For pulse echo method: Maximum testable length = ½ x Velocity x Clock interval Typically the Clock interval should be 5 time the transit time, i.e. the sound path should travel 5 times the maximum testable length. (1st BWE, 2nd BWE, 3rd BWE, 4th BWE to 5th BWE.) Note: The Clock interval has neglected the time occupied by each pulse.

Pulse Repetition Rate and Penetration

Pulse Repetition Rate and Penetration

Pulse-Length and Near Surface Sensitivity

Q186: The maximum scanning speed possible is primarily determined by: A. B. C. D.

The frequency of transducer Viscous drag problem The pulse repetition rate of test instrument The persistency of the ultrasonic instrument display

Q200: When setting up an ultrasonic inspection, the repetition frequency for the ultrasonic instrument should be set: A. B. C. D. E.

So that its period is at least as long as the operating time The same as the transducer resonance frequency As low as possible to avoid over-pulsing and distortion According to the instrument manual None of the above

5.12: Interferences & Non-Relevant Indications Following are signal interferences that may produce non-relevant UT indications: 1. 2. 3. 4. 5. 6. 7.

Electrical interference Transducer interference Test specimen geometric interference Test specimen surface interferences Test material structure interferences Test material internal mode conversion interference UT techniques induced interferences (In correct PRR/ Band width/ Frequency selection/ Excessive Beam Spread/ etc.)

Transducer Interference- Transducer internal reflections & Mode conversion may cause interference

Specimen Surface Interference Excessive surface roughness, air bubbles on the surface (on the transducer front, specimen front and back for immersion techniques. Surface wave for testing near the edges

Specimen Surface Interference

? ?

Specimen Surface Interference- You can determined whether the signal is from the surface wave or the refracted wave simply by touching the surface ahead of the wave (assuming the velocity of surface wave at 0.9 of the shear wave)

Mode Conversion Interference The mode conversion interference during testing of long cylindrical specimen with longitudinal wave often appeared after the first back wall echo. The signal can be easily distinguished and ignore.

Material Geometric Interference False signals may generated due to the test specimen structural configurations resulting in spurious signals.

Non Relevant Indications Transducer with Excessive Beam Spread may generate signal, usually after the 1st BWE. The example below the convex surface defocused the beam and lead to excessive beam spread, using a proper contoured probe may eliminate the problem. However excessive contour may results in generation of surface wave.

Non Relevant Indication Large grain size especially casting may cause excessive hash or grass signal. Properly selecting probe with lower frequency may relieve the problem. However this can only de accomplished with reduction in sensitivity.

Non Relevant Indication Large grain size at heat affected zone HAZ (CGHAZ) may cause localized signal due to large grain size. The signal may be wrongly assessed as a defect.

Non Relevant Indications The geometric abnormalities at root penetration and weld surface (crown) may reflect the sound path, returning to the receiver as signals. To distinguished the non relevant indications, finger touching will damped the signals. Further testing may be necessary to ensure the signals were not from the surface defects like surface crack. Any near surface indication that are unusually consistent could be a non relevant indication.

5.13: Entry Surface Variables Entry surface variables include: 1. surface roughness 2. surface coatings 3. couplant condition. 5.13.1 Surface Roughness Surface roughness will have several possible effects on the inspection of a test piece. In contact testing roughness on a gross scale results from: weld spatter, plate scale, dirt (sand) and rough cast surfaces from sand casting. These irregularities will cause some points of contact to push away the couplant and force it into the lower areas around the probe. If the couplant is not sufficiently viscous it will drain away quickly and fail to couple the probe to the test piece. See Figure 8-3.

http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm

Entry surface variables: Surface roughness

Air Gap Low Viscosity Couplant

High Points of Rough Surface

In addition to reduced coupling, which will reduce signal amplitudes, the rough surface increases the rate of wear on the probe. On an otherwise smooth surface isolated protrusions such as weld spatter can hinder or stop probe motion or in the case of mechanized systems there may be sufficient force to move the probe past the obstruction but this could result in damaging the probe by either tearing it from its mounting or severely scoring the plastic wedge. When the dirt on the test piece is very fine (similar to a flour texture) coupling can be prevented due to surface tension preventing the liquid couplant penetrating to the metal. Unless a transfer value has been established between test piece and calibration piece, this could go undetected. In addition to affecting coupling, surface roughness tends to reduce signal amplitude by scattering and focusing the beam. This applies to both contact and immersion testing.

Whether uniform or irregular, a rough surface has the potential to present a scattering effect at an interface where a beam impinges. The degree of scattering is based on the ratio of roughness to wavelength. When roughness is less than about 1/10 a wavelength, scatter will be negligible. To reduce signal losses due to scattering an operator can select a lower frequency probe. With a wavelength of 0.37mm in water for a 4MHz probe, signal loss due to scatter can occur for irregularities as small as about 0.04mm. In addition to signal reduction another effect of surface irregularities is to redirect and mode convert some energy which when returned to the probe can be the source of spurious signals. In contact testing false indications from standing waves resulting from scatter on rough surfaces will normally have short sound paths. They can be eliminated as true flaws by failing to locate any trace of indication from the full skip or from the opposite side.

Unless done properly, removal of surface roughness by mechanical means can result in further scattering problems. Small curved gouges left by a grinding wheel used to remove spatter or machining grooves can form small lenses. The affect of grinding can be unpredictable. Some of the lensing may concentrate the beam thereby increasing signal amplitude, or, the lens effect may be a de-focusing of the beam, again resulting in lower than expected signal amplitudes. Uniform surface preparation by sand or shot blasting usually provides a good surface for ultrasonic testing. Removal of excess metal by a hand held grinding wheel is commonly used on weld caps and roots. When a pipe weld has had its root ground flush and inspection can only be performed from the outside diameter, quality of grinding can result in unnecessary repair calls if grinding has been along the weld axis. The small grooves made by the grinding wheel run parallel to the root edge and are easily confused with lack of fusion, missed edge or undercut defects.

Keywords on Rough Surface: 1. The degree of scattering is based on the ratio of roughness to wavelength. When roughness is less than about 1/10 a wavelength, scatter will be negligible. 2. Consequences of Surface Roughness:  Signal reduction  Redirect and mode convert some energy which when returned to the probe can be the source of spurious signals. 3. The False Indications: In contact testing false indications from standing waves resulting from scatter on rough surfaces will normally have short sound paths. They can be eliminated as true flaws by failing to locate any trace of indication from the full skip or from the opposite side

5.13.2 Surface Coatings Surface coatings are added to protect a surface from corrosion or to enhance its appearance. Thin films, such as oxide layers, anodizing layers or electroplated finishes, and the slightly thicker coatings of paint or lacquer are usually well bonded to the surface. Quality of bond may be compared to the uncoated reference block by a simple transfer value. Even a slight loss due to the coating may be preferable to removing the coating and trying to inspect on the rough surface it hides. When thickness testing is done on a painted surface the paint thickness can add error to the reading. For example: A nominal 25mm steel plate has a cellulose paint coating of 0.5mm. Vsteel = 5980m/s, V paint = 2600m/s. If a digital thickness meter is calibrated on a 25mm thick piece of the steel plate without the paint coating and then placed on the painted surface an error will occur.

The coating is sufficiently thin that its interface with the metal will occur in the dead zone but the duration of time spent in the paint is added to the travel time to the opposite wall of the plate. If the true plate thickness at the point of measurement is 25.16mm and the paint coating is 0.5mm thick, the time in the paint is 0.5/ 2.6 x 106 = 0.19µs. 0.19 microseconds is equivalent to 1.15mm in steel. The reading on the digital meter would combine the two thickness as though all travel was in steel. This results in 25.16 + 1.15 = 26.31mm as the indicated thickness. This problem can be overcome by using an A-scan display and measuring the interval between the first and second echo instead of the main bang and first echo. This is shown in Figure 8-4.

Thickness Measurement with Surface Coating Figure 8-4.

5.13.3 Couplant Condition Both contact and immersion methods utilize intervening media to transfer sound from the probe into the test piece and back to the receiver. With immersion methods it is accomplished by a single fluid medium. In contact testing there are nearly always at least two intervening media; the delayline or protective face and the thin film of coupling fluid or grease. Attenuation and acoustic velocity are the two main properties that dictate the performance of a couplant. Attenuation affects amplitude of the signal and velocity will determine both transit time and refracted angles. But attenuation and velocity of couplants are not independent properties. Each is a function of other parameters. Unless these parameters are controlled or in some way compensated for, gross variations from the reference value or calibration conditions can result.

Attenuation of couplants varies with material composition as would be expected. Published attenuation values are available for many materials as indicated in the table below. Attenuation coefficients are often quoted in Neper which allow for frequency dependence. 1 Np = 8.686 dB. Attenuation per unit length= Attenuation Coefficient x f 2 x 8.686 dB/cm. Table 8-1 indicates Attenuation Coefficient of some common liquids. -15

In more practical terms, for water with longitudinal wave of 500KHz this would mean an attenuation of about 5 dB per meter. Example: Attenuation factor for water = 25.3x 10-15 Neper Frequency= 0.5MHz The attenuation = Attenuation Coefficient x f 2 The attenuation = 25.3 x 10-15 x (0.5x106)2 Neper/ cm The attenuation = 25.3 x 10-15 x (0.5x106)2 x 8.686 = 0.055 dB/cm or 5.5 dB/m Since such long water path lengths are not normally used the 0.005 dB/mm attenuation is considered negligible. But for the heavier oils attenuations 200 to 500 times greater can have significant effects on signal amplitude and frequency content. For the fixed delay-lines or wedge materials used in contact testing attenuation variations can be far more pronounced and variation between manufacturers can cause considerable response differences.

Table 8.2

For example the plastics listed in table 8-2 are typical wedge materials selected by manufacturers and based on velocity for refraction purposes, but attenuation differences would result in noticeable amplitude response variation and frequency content of transmitted waveforms. Since the operator rarely knows what wedge material a manufacturer has used, little can be done to correct for potential variations in periodic inspections where results of tests taken with one or more years separation are compared.

For example the plastics listed in table 8-2 are typical wedge materials selected by manufacturers and based on velocity for refraction purposes, but attenuation differences would result in noticeable amplitude response variation and frequency content of transmitted waveforms. Since the operator rarely knows what wedge material a manufacturer has used, little can be done to correct for potential variations in periodic inspections where results of tests taken with one or more years separation are compared.

5.13.4 More Reading: What is Neper ? The Neper (unit symbol Np) is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. The unit's name is derived from the name of John Napier, the inventor of logarithms. As is the case for the decibel and Bel, the Neper is unit of the International System of Quantities (ISQ), but not part of the International System of Units (SI), but it is accepted for use alongside the SI. The Neper is a natural linear unit of relative difference, meaning in Neper (logarithmic units), relative differences add, rather than multiply. This property is shared with logarithmic units in other bases, such as the Bel.

http://en.wikipedia.org/wiki/Neper

Like the decibel, the Neper is a unit in a logarithmic scale. While the Bel uses the decadic (base-10) logarithm to compute ratios, the Neper uses the natural logarithm, based on Euler's number (e ≈ 2.71828). The value of a ratio in Neper is given by

where x1 and x2 are the values of interest, and ln is the natural logarithm. In the ISQ, the Neper is defined as 1 Np = 1. 1 Np = ln (2.718) when the ration of

= 2.718

The neper is defined in terms of ratios of field quantities (for example, voltage or current amplitudes in electrical circuits, or pressure in acoustics), whereas the decibel was originally defined in terms of power ratios. A power ratio 10 log r dB is equivalent to a field-quantity ratio 20 log r dB, since power is proportional to the square (Joule's laws) of the amplitude.

Hence the Neper and dB are related via: The decibel and the Neper have a fixed ratio to each other. Example: For a ratio of (x1/x2) The (voltage) level ratio is:

Hence:

Q31: Rough surfaces can cause undesirable effects which are noticeable when parts are tested ultrasonically, include: A. Annular maximum rings B. An increase in width of front face echo and consequent loss of resolving power C. Acoustic mismatch D. Asymmetrical modes Q32: Rough surfaces cause echo amplitude from discontinuities within the part to: A. B. C. D.

Increase Decrease Not change Change frequency

5.14

The Concept of Effective Distance

Effective distance in ultrasonic testing is a distance which take into account of the required sound beam overlap and hits. The effective distance always smaller in the calculation. Keywords: Loop or full path = Thickness x 2 Effective Distance Deff = Beam Diameter / overlap, or Effective Distance Deff = Beam Diameter / No. of hits Effective Distance Deff = Loop x hits T = Time (of interest) T = Deff / Velocity PRR = 1/T RPM = Revolution per minute Maximum linear traversing speed V for effective inspection = RPM x Deff

Scanning Speed: Scanner speed = (PRR × Effective diameter of probe / Number of hits) Speed of test part = (PRR × Effective diameter of probe / Number of hits) Where: Effective dia. of probe = Dia. of probe – 2 [ (Dia. of probe) × (Percent of overlap between scan / 100) ] Linear speed of disc or pipe in mm/ s = (2πr x RPM / 60)

Q&A on The Concept of Effective Distance

e c n a st i D

Q1: A tubular product is tested by AUT (or UT). The tube is rotated at 500 RPM If beam diameter is 10 mm and overlap between scan is 50%. Calculate maximum length of the tube that can be tested. Answer: The effective distance covered by each revolution (Deff) is 5mm. For 500 RPM the total distance covered: 500 x 5 mm = 2500 mm The inspection rate = 2500/ minute#

Effective coverage 50% overlap = 5mm

Beam diameter (coverage) =10mm

Q2: A steel bar with 200 mm thick is scanned by UT. Minimum number of hits required is 10. What is the maximum PRR to avoid the ghost echo. (VL, steel = 0.59 cm/μs ) Answer: The total distance to travel Deff = Thickness x [2 x 10 hits] = 400cm = 4m The time taken to complete the 10 loops (T) = [effective distance / Velocity] T= 4/ 5900 =1/1475 The maximum pulse repetition rate PRR = 1/T = 1475

Distance travelled by 10hits= 10x 200x2=4m

10 hits

200mm

Q3: Minimum number of hits required is 2. What is the maximum allowable axial speed for a probe with effective diameter of 102 mm and PRR of 800 Hz. Answer: The diameter of the beam = 102mm The effective distance by one pulse, Deff = Beam Diameter / No of hits Deff = 102/2 = 51mm The total distance covered 800 pulse = 51x800 =40.8m The axial speed is 40.8m/s

Q4: Assuming that Minimum 3 pulses are required to trigger the alarm in AUT (or UT) . What would be (maximum) scanning speed to detect 0.5 mm size defect while using 4 MHz probe, 10 mm beam diameter with PRR of 0.5 kHz. Answer: The distance Covered by scan in 1 second = PRR x Diameter = 500 x 10 = 5m/s The effective distance covered = D/ hits = 5/3 = 1.667m/s Hint: the size of the defect and mode frequency has no implication of the calculation

Q5: What is the maximum PRR is needed for contact test of steel material with 100 mm thick using longitudinal wave. Minimum number of hits required is 10. (VL, steel = 0.59 cm/μs ) Answer: Distance = 100 x 2 =200mm for single loop. Effective distance Deff = 200 x10 =2000 mm Time = Deff / Velocity = 1/2950 PRR = 1/T = 2950

Q6: What is the maximum PRR is needed for immersion testing of aluminum with 80 mm thick using longitudinal wave. The water path is 10 mm. Minimum number of hits required is 20. (VL, aluminum = 6.32 × 103 m/s, VL, water = = 1.48 × 103 m/s) Answer: Single Water path =10mm, Single Al path= 80mm LoopH2O = 0.02 m, loopAl = 0.16 m Time for traversing single hit = [0.02/1480] + [0.16/6320] = 3.883 x 10-5 s Time for traversing 20 hits T’ = 20 x T = 3.883s PRR = 1/T’ = 1287

Q7: A steel plate size 6.2 m ×1.8 m ×0.1m is scanned using 25 mm diameter normal probe and overlap between scan is 20%. Minimum number of hits required is 15. Calculate the inspection time if scanning speed is 500 mm/s. Answer: (Standard Answer) Effective area of probe = 25 x0.8 x 500 = 10000 mm2 Area of plate = 6200 x 1800 mm2 T = (6200 x 1800)/ 10000 = 1116 s

20% overlap

25mm Effective area = 25 x 500 x 0.8 = mm2

Scanning speed 500 mm/s

Q7: A steel plate size 6.2 m ×1.8 m ×0.1m is scanned using 25 mm diameter normal probe and overlap between scan is 20%. Minimum number of hits required is 15. Calculate the inspection time if scanning speed is 500 mm/s. Answer: In-correct answer. Area covered by probe in 1 second = 25 x 500 = 12500 mm2 Effective plate area = 6200 x 1800 x 1.2 mm2 Time to scan the plate = [6200 x1800 x 1.2 / 12500] = 1071.36s (maintaining the probe area, increase the surface area by the overlap) Hint: the plate thickness is of no concern.

Q8: Assume that the minimum PRR is needed for contact test of a given steel plate is 240 Hz. How much volume would be covered under test if the PRR is set at 120 Hz. Answer: Volume inspected by ultrasound per unit time = PRR x Pulse length x Cross sectional area of beam at point of interest. = PRR x constant (k) Where k = Pulse length x Cross sectional area of beam at point of interest The ration of volume covered = 120k / 250k x 100 = 50%

5.15: Questions & Answers Exercises

The 6 dB Method For Large Reflector (greater than beam width), i.e. there is no BWE.

Compared 6 dB Drop Sizing with Equalization Technique The Equalization Back Wall Sizing- The probe moving off the edges of the reflector until the amplitude is equal to the rising BWE

Q1 What is the correct water path between the transducer and the steel front surface to focused a transducer for a area of interest at ½ below a steel surface? Given that: Focal length of transducer in water = 6” Velocity of sound in water= 1484 m/s Velocity of sound in steel = 5920 m/s Equivalent depth in water for ½ steel depth = 4x ½ = 2” The water path= 6”- 2” = 4”

Q5: Ultrasonic inspection is being for a circumferential weld of a pressure vessel. Equipment is calibrated at the beginning of the examination and scanning started at 9.00 AM. In between calibration is also done and at 12.00 noon the equipment is not functioning properly. Still 30 % of weld is to be examined. As per procedure and as level II one can: A. go ahead with the scanning by doing recalibration B. perform the inspection fully from the beginning after recalibration C. bring another equipment and proceed scanning from the left out place D. check and recalibrate the equipment and continue scanning from the portion where scanning started after calibration at 11.00 AM

Q5: When dissimilar metal welds is to be tested ultrasonically and scanning is to be performed from both sides of the weld the calibration blocks shall be made from: A. B. C. D.

material having higher tensile strength both the materials material subjected to heat treatment calibration block material is aluminum

Break Time

mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/ 39440/v0001/reflector:20587?BBCUID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f556 74c540f1856834&SSO2-UID=

Section 6: Selected Applications & Techniques

Content: Section 6: Selected Applications & Techniques 6.1: Defects & Discontinuities 6.2: Rail Inspection 6.3: Weldments (Welded Joints) 6.4: Pipe & Tube 6.5: Echo Dynamic 6.6: Technique Sheets 6.7: Material Properties-Elastic Modulus Measurements 6.8: High Temperature Ultrasonic Testing 6.9: Thickness Gauging 6.10: In-Service Inspection Continues next page….

6.11: 6.12: 6.13: 6.14: 6.15: 6.16:

Casting Inspection of bonded Joints Corrosion Monitoring Crack Monitoring Residual Stress Measurements Bond Testing

Appendix: (Non-exam) 6.App-1: TOFD Introduction

6.1: Defects & Discontinuities

6.1.1 Casting Defects & Discontinuities

Casting Defects & Discontinuities

Casting Defects & Discontinuities- A Cold Shut is caused when a molten metal is poured over solidified metal without fusing.

Casting Defects & Discontinuities – Hot tear or shrinkage crack forms when the molten section of unequal thickness solidified and the shrinkage stress tear the partially molten apart.

Casting Defects & Discontinuities

Micro-shrinkage is usually many small subsurface holes that appear at the gate of casting / can also occur when molten metal must flow from a thin section into thicker section of casting. Blow hole are small hole at the surface of the casting caused by gas which comes from the mold itself. (wet sand mould forming steam resulting in blowhole) Porosity is caused by entrapped gas. It is usually subsurface or surface depending on the mold design.

Casting Defects & Discontinuities

Casting Defects & Discontinuities- Hot Tear

Casting Defects & Discontinuities- Blister

Casting Defects & Discontinuities- Porosity

Casting Defects & Discontinuities- Porosity

Casting Defects & Discontinuities- Porosity

Casting Defects & Discontinuities- Porosity

Casting Defects & Discontinuities - Mismatch

Casting Defects & Discontinuities- Cold Shut

Casting Defects & Discontinuities- Missrun

Casting Defects & Discontinuities- Misrun

Casting Defects & Discontinuities- Blow Hole

Casting Defects & Discontinuities- Gas Porosity

Casting Defects & Discontinuities- Porosity

Casting Defects & Discontinuities- Cold Shut

Casting Defects & Discontinuities- Shrinkage Cavity

Casting Defects & Discontinuities- Assorted

6.1.2 Processing Defects & Discontinuities

Processing Defects & Discontinuities

Salute to the Steel Workers!

Processing Defects & Discontinuities- Lamination formed when the casting defects are flatten during rolling, forging, extrusion or other mechanical working processes.

Processing Defects & Discontinuities- Stringers formed when the billet is rolled into shape the casting non metallic inclusions are squeezed into long and thinner inclusions.

Processing Defects & Discontinuities- Forging lap is caused by folding of metal on the surface, usually when some of the metal is squuaed ot between the two dies.

Processing Defects & Discontinuities- Forging burst is a rupture causes by forging at improper temperature. The burst may be internal or external.

Processing Defects & Discontinuities

Q9: The preferred method of ultrasonically inspecting a complex-shape forging: A. Is an automated immersion test of the finished forging using instrument containing a calibrated attenuator in conjunction with a C-scan recorder B. Combined thorough inspection of the billet prior to forging with a careful inspection of the finished part in all areas where shape permit C. Is a manual contact test of the finished part D. Is an automated immersion test of the billet prior to forging

6.1.3 Welding Defects & Discontinuities

Welding Defects & Discontinuities

Welding Defects & Discontinuities

Welding Defects & Discontinuities

Welding Defects & Discontinuities

Welding Defects & Discontinuities

Welding Defects & Discontinuities

Welding Defects & Discontinuities- Incomplete Penetration

Welding Defects & Discontinuities- Slag Inclusion

Welding Defects & Discontinuities- Cluster Porosity

Welding Defects & Discontinuities- Lack of Sidewall Fusion (with Slag entrapped)

Welding Defects & Discontinuities- Wagon Track (slag inclusion at hot pass)

Welding Defects & Discontinuities- Burn Thru

Welding Defects & Discontinuities- Offset with LOP

Welding Defects & Discontinuities- Excessive Penetration

Welding Defects & Discontinuities- Internal (Root) Under Cut

Welding Defects & Discontinuities- Transverse Crack

Welding Defects & Discontinuities- Tungsten Inclusion

Welding Defects & Discontinuities- Root Pass Porosity

6.1.4

Service Induced Defects & Discontinuities

Service Induced Defects & Discontinuities

http://failure-analysis.info/2010/05/analyzing-material-fatigue/

Service Induced Defects & Discontinuities- Fatigue Cracks

Figure 4-24 – In a carbon steel sample, metallographic section through a thermal fatigue crack indicates origin at the toe of an attachment weld. Mag. 50X, etched.

Figure 4-26 – Metallographic cross-section of a superheated steam outlet that failed from thermal fatigue. Unetched.

Figure 4-36 – Weld detail used to join a carbon steel elbow (bottom) to a weld overlaid pipe section (top) in high pressure wet H2S service. Sulfide stress cracking (SSC) occurred along the toe of the weld (arrow), in a narrow zone of high hardness.

Figure 4-37 – High magnification photomicrograph of SSC in pipe section shown in Figure 4-36.

Figure 4-38 – Failure of DMW joining 1.25Cr-0.5Mo to Alloy 800H in a Hydrodealkylation (HAD) Reactor Effluent Exchanger. Crack propagation due to stresses driven at high temperature of 875°F (468°C) and a hydrogen partial pressure of 280 psig (1.93 MPa).

Figure 4-57 – Vibration induced fatigue of a 1-inch socket weld flange in a thermal relief system shortly after startup.

Figure 4-58 – Cross-sectional view of the crack in the socket weld in Figure 457.

Figure 5-1 – Localized amine corrosion at the weld found in piping from reboiler to regenerator tower in an MEA unit. Many other similar cases found, some going as deep as half thickness. They were originally found and mistaken as cracks with shear wave UT inspection.

Figure 5-2 – Hot Lean Amine Corrosion of Carbon Steel:

Figure 5-3 – Preferential weld corrosion in lean amine (Reference 5)

Figure 5-46 – Overhead interstage knockout drum vapor outlet nozzle.

Figure 5-47 – Carbonate cracking adjacent to a weld (Reference 6).

Figure 5-48 – Metallographic sample showing intergranular carbonate cracking developed after 6 months service (Reference 6).lean amine (Reference 5)

Figure 5-49 – Most cracks originate in base metal but this weldment contained a crack that originated at the root and propagated through the weld metal. Other cracks appear to have initiated in the HAZ (Reference 7).

6.2: Rail Inspection

Rail Inspection One of the major problems that railroads have faced since the earliest days is the prevention of service failures in track. As is the case with all modes of high-speed travel, failures of an essential component can have serious consequences. The North American railroads have been inspecting their most costly infrastructure asset, the rail, since the late 1920's. With increased traffic at higher speed, and with heavier axle loads in the 1990's, rail inspection is more important today than it has ever been. Although the focus of the inspection seems like a fairly well-defined piece of steel, the testing variables present are significant and make the inspection process challenging. Rail inspections were initially performed solely by visual means. Of course, visual inspections will only detect external defects and sometimes the subtle signs of large internal problems.

The need for a better inspection method became a high priority because of a derailment at Manchester, NY in 1911, in which 29 people were killed and 60 were seriously injured. In the U.S. Bureau of Safety's (now the National Transportation Safety Board) investigation of the accident, a broken rail was determined to be the cause of the derailment. The bureau established that the rail failure was caused by a defect that was entirely internal and probably could not have been detected by visual means. The defect was called a transverse fissure (example shown on the bottom). The railroads began investigating the prevalence of this defect and found transverse fissures were widespread.

Transverse Fissure

Transverse Fissure

Transverse Fissure

One of the methods used to inspect rail is ultrasonic inspection. Both normal- and angle-beam techniques are used, as are both pulse-echo and pitch-catch techniques. The different transducer arrangements offer different inspection capabilities. Manual contact testing is done to evaluate small sections of rail but the ultrasonic inspection has been automated to allow inspection of large amounts of rail. Fluid filled wheels or sleds are often used to couple the transducers to the rail. Sperry Rail Services, which is one of the companies that perform rail inspection, uses Roller Search Units (RSU's) comprising a combination of different transducer angles to achieve the best inspection possible. A schematic of an RSU is shown below.

Techniques: Wheel Probe

Techniques: Examples of axles with outside bearings of the Deutsche Bundesbahn. (a) Of goods truck; (b) axle with roller bearing, bearing ring not removed; c same with additional brake disc

Techniques: (c) same with additional brake disc

6.3: Weldments (Welded Joints)

6.3.1: UT of Weldments (Welded Joints) The most commonly occurring defects in welded joints are porosity, slag inclusions, lack of side-wall fusion, lack of inter-run fusion, lack of root penetration, undercutting, and longitudinal or transverse cracks. With the exception of single gas pores all the defects listed are usually well detectable by ultrasonics. Most applications are on low-alloy construction quality steels, however, welds in aluminum can also be tested. Ultrasonic flaw detection has long been the preferred method for nondestructive testing in welding applications. This safe, accurate, and simple technique has pushed ultrasonics to the forefront of inspection technology. Ultrasonic weld inspections are typically performed using a straight beam transducer in conjunction with an angle beam transducer and wedge. A straight beam transducer, producing a longitudinal wave at normal incidence into the test piece, is first used to locate any laminations in or near the heataffected zone. This is important because an angle beam transducer may not be able to provide a return signal from a laminar flaw.

UT of Weldments (Welded Joints)

a = s sinß

F

a' = a - x

s

d' = s cosß 0

20

40

60

80

100

d = 2T - t'

a a'

x ß Work piece with welding

ß = probe angle s = sound path a = surface distance a‘ = reduced surface distance d‘ = virtual depth d = actual depth T = material thickness

s

Lack of fusion

d

UT Calculator

Flaw Detection- Depth Determination

The second step in the inspection involves using an angle beam transducer to inspect the actual weld. Angle beam transducers use the principles of refraction and mode conversion to produce refracted shear or longitudinal waves in the test material. [Note: Many AWS inspections are performed using refracted shear waves. However, material having a large grain structure, such as stainless steel may require refracted longitudinal waves for successful inspections.] This inspection may include the root, sidewall, crown, and heataffected zones of a weld. The process involves scanning the surface of the material around the weldment with the transducer. This refracted sound wave will bounce off a reflector (discontinuity) in the path of the sound beam. With proper angle beam techniques, echoes returned from the weld zone may allow the operator to determine the location and type of discontinuity.

T

= Plate Thickness

ϴ = Shear wave angle LEG = T/Cos ϴ, V path= 2 x LEG. Skip = 2.T Tan ϴ

https://www.mandinasndt.com/index.php?option=com_content&view=article&id=32%253A ut-angle-beam-calculator&catid=12%253Atools&Itemid=18 https://www.nde-ed.org/GeneralResources/Formula/AngleBeamFormula/AngleBeamTrig.htm

Flaw Detection- Triangulations of reflector ϴ = Refracted angle

T= Thickness

V PATH= 2x LEG= 2T/Cos ϴ

ϴ

LEG1=LEG2= T/Cos ϴ SKIP= 2.T Tan ϴ

Flaw Detection- Triangulations of reflector ϴ = Refracted angle

T= Thickness

Depth= S.Cos ϴ

ϴ

Surface Distance= S.Sin ϴ

To determine the proper scanning area for the weld, the inspector must first calculate the location of the sound beam in the test material. Using the refracted angle, beam index point and material thickness, the V-path and skip distance of the sound beam is found. Once they have been calculated, the inspector can identify the transducer locations on the surface of the material corresponding to the crown, sidewall, and root of the weld.

6.3.2 Weld Scanning

Expert at works

Typical Scanning Patterns: Typically the weld should be inspected in the 1st or 2nd leg (1st Skip).

Typically scanning patterns

Weld Scanning

Weld Scanning

Weld Scanning

Weld Scanning

Echo Dynamic- Position of Defects Sometimes it will be possible to differentiate between these 2 defects simply by plotting their position within the weld zone:

Echo Dynamic- Position of Defects

Plate Weld Scanning

Plate Weld Scanning

Plate Weld Scanning

Plate Weld Scanning

Plate Weld Scanning

Practice Makes Perfect 52. One of the most apparent characteristics of a discontinuity echo, as opposed to a non-relevant indication is: (a) Lack of repeatability (b) Sharp, distinct signal (c) Stable position with fixed transducer position (d) High noise level 58. What useful purpose may be served by maintaining grass on the baseline? (a) To estimate casting grain size (b) To provide a reference for estimating signal to noise ratio (c) To verify adequate coupling to the test piece (d) All of the above

Practice Makes Perfect 62. Which of the following conditions would be most likely to cause strong, interfering surface waves? (a) High frequency transducers (b) Testing on a small diameter surface (c) Testing on a flat surface (d) Testing on a curved surface with a contoured wedge and transducer

6.4: Pipe & Tube

Pipe & Tube

Pipe & Tube

Experts at work

Pipe Scanning

Pipe Scanning

Pipe Scanning

48.59o max

30o max

Pipe Scanning

Pipe Scanning

Pipe Scanning- thickness/OD ratio

Pipe Scanning- thickness/OD ratio When the t/OD ratio = .2 , t=.2OD, ID=OD-2t= OD-.4OD= .6OD ϴ max = Sin-1(ID/OD), ϴ max = Sin-1(0.6), ϴ max = 37° Max. For the sound path to scans the inner face the maximum shear angle shall be 37° Max. Therefore 45° /60° /70° probe can not scan the pipe inner face.

Pipe Scanning- Contact Methods

Pipe Scanning- Contact Methods

Pipe Scanning- Contact Methods

Q: Calculate the maximum shear wave angle and the range for 360° revolution scanning when the shear wave angle is 45°. Given that the OD=6” Thickness=3/4” Answer: (a) The maximum shear wave angle ϴ = Sin-1(ID/OD) = Sin-1(2.25/3) ϴ = 48.6° Max. (b) ?

Answer part B a/Sin A = b/Sin B b

2.25/ Sin 45 = b / Sin B, 3.182= b/ Sin B, c = a.Sin B, Sin B= c/a

c a

3.182= b/c x 2.25, b/c= 1.414

Q35: During immersion testing of pipe or tubing the incident longitudinal wave angle must be limited to a narrow range. The reason for the upper limit is: (a) To avoid complete reflection of ultrasound from the test piece (b) To prevent formation of Rayleigh waves (c) To prevent formation of shear waves (d) To avoid saturating the test piece with ultrasound

Q35: Which of the following may result in a narrow rod if the beam divergence results in a reflection from a side of the test piece before the sound wave reaches the back surface: A. B. C. D.

Multiple indications before the first back reflection Indications from multiple surface reflections Conversion from longitudinal mode to shear mode Loss of front surface indications

6.5: Echo Dynamic

Expert at works

6.5.1

Basic echodynamic pattern of reflectors

Echo Dynamic of Discontinuity- Non-destructive testing of welds Ultrasonic testing - Characterization of indications in welds; German version EN 1713:1998 + A1:2002

Basic echodynamic pattern of reflectors C.1 Pattern 1 Point-like reflector response, figure C.1. At any probe position the A-scan show a single sharp echo. As the probe is moved this rises in amplitude smoothly to a single maximum before falling smoothly to noise level.

4 5 3 2 1

6 7

C.1 Pattern 1 Point-like reflector

C.1 Pattern 1 Point-like reflector

C.2 Pattern 2 Extended (elongated) smooth reflector respond, figure C.2. At any probe position the A-scan shows a single sharp echo. When the ultrasound beam is moved over the reflector the echo rises smoothly to a plateau and is maintained with minor variation in magnitude up to 4 dB, until the beam moves off the reflector, when the echo fall smoothly to noise level.

C.2 Pattern 2 Extended (elongated) smooth reflector

C.2 Pattern 2 Extended (elongated) smooth reflector

C.2 Pattern 2 Extended (elongated) smooth reflector (figure modified to depict obliquely oriented planar face)

Extended (elongated) smooth reflector-planar face obliquely oriented

C.3 Pattern 3 Extended (elongated) rough reflector response. There are two variants of this pattern, depending upon the angle of incident of the probe beam on the reflector.

C.3 Pattern 3a Extended (elongated) rough reflector response. Near normal incidence, figure C.3a At any probe position the A-scan shows a single but rugged echo. As the probe moved this may undergo large (>+/- 6dB) random fluctuation in amplitude. The fluctuation are caused by reflection from the different facets of the reflector and by interference of waves scattered from the groups of facets.

C.3 Pattern 3a Extended (elongated) rough reflector response.

C.3 Pattern 3a Extended (elongated) rough reflector response.

C.3 Pattern 3b Oblique incidence, travelling echo pattern, figure C.3 b At any probe position, the A-scan shows an extended train of signals (subsidiary peaks) within a bell-shaped pulse envelope. As the probe is moved each subsidiary peak travels through the pulse envelop, rising to its own maximum toward the center envelop and then falling. The overall signal may shown large (>+/-6dB) random fluctuation in amplitude.

C.3 Pattern 3b Oblique incidence, travelling echo pattern

C.3 Pattern 3b Oblique incidence, travelling echo pattern

C.4 Pattern 4 Multiple reflector respond, figure C.4. At any probe position the A-scan shows a cluster of signal which may or may not be well resolved in range. As the probe is moved the signals rise and fall at random but the signal from each separate reflector element ,if resolved, shows pattern 1 respond.

C.4 Pattern 4 Multiple reflector respond

C.4 Pattern 4 Multiple reflector respond

Echodynamic- Change of echo height and echo shape when the direction of irradiation is changed. (a) On flat or linear flaw; (b) on rounded flaw

Echodynamic- Differences between the indications of inclusions and cracks, drawn schematically and exaggerated for greater clarity. a Inclusions; b flake cracks. The echoes of the more distantflaws, because of divergence and attenuation of the sound beam, are rather weak

Break Time

Echo Dynamic of Discontinuity- Flaw detection

Echo Dynamic of Discontinuity- Flaw Detection

Echo Dynamic of Discontinuity- Flaw detections

Echo Dynamic of Discontinuity- Improper flaw orientation

Echo Dynamic of Discontinuity- Improper flaw orientation

Echo Dynamic of Discontinuity- Reflection angle

Echo Dynamic of Discontinuity- Angles of reflection

Echo Dynamic of Discontinuity- Improper flaw orientation

Echo Dynamic of Discontinuity- Perfect flaw orientation

Echo Dynamic of Discontinuity- Improper flaw orientation

Echo Dynamic of Discontinuity- Vertical near surface flaw

Echo Dynamic of Discontinuity- Tandem Techniques

Echo Dynamic of Discontinuity- Tandem Techniques

Echo Dynamic of Discontinuity- Tandem Techniques

Echo Dynamic

Echo Dynamic- Root Concavity

Echo Dynamic

Echo Dynamic

Echo Dynamic

Echo Dynamic

Echo Dynamic

Crack

Echo Dynamic- Broad indication with low amplitude

Echo Dynamic- Shaper indication and higher amplitude than porosity

Echo Dynamic

Echo Dynamic Threadlike defects, point defects and flat planar defects orientated nearnormal to the beam axis all produce an echo response which has a single peak

Echo Dynamic The echo response from a large slag inclusion or a rough crack is likely to have multiple peaks:

Echo Dynamic In case “a” it will be difficult to determine whether the defect is slag or a crack. “Rotational- Swivel” or “orbital” probe movements may help:

Echo Dynamic Typical Echo Dynamic Patterns

Echo Dynamic Typical Echo Dynamic Patterns

Echo Dynamic Typical Echo Dynamic Patterns

Q. A smooth flat discontinuity whose major plane is not perpendicular to the direction of sound propagation may be indicated by: A. B. C. D.

An echo amplitude comparable in magnitude to the back surface reflection A complete loss of back surface reflection An echo amplitude larger in magnitude than the back surface reflection All of the above

Q183. In immersion testing, irrelevant or false indications caused by contoured surfaces are likely to result in a: A. B. C. D.

Broad base indication Peaked indication Hashy signal Narrow based indication

Q24. During inspection of a parallel sided machined forging using straight beam immersion techniques, a diminishing back reflection in a localized area in the absence of a defect indication would least likely represent: A. B. C. D.

A course grain structures A small non-metallic stringer A defect oriented at a severe angle to the entry surface A large inclusion.

Q46. Which best describes a typical display of a crack whose major surface is perpendicular to the ultrasound beam? A. B. C. D.

A broad indication A sharp indication A indication will not show due to improper orientation A broad indication with high amplitude

Q46. A smooth flat discontinuities whose major plane is not perpendicular to the direction of sound propagation may be indicated by: A. B. C. D.

An echo amplitude comparable in magnitude to the back surface reflection A complete loss of back surface reflection An echo amplitude larger in magnitude than the back surface reflection All of the above

6.6: Technique Sheets

Expert at works

Hanger Pin Testing using Shear Wave http://www.fhwa.dot.gov/publications/research/infrastructure/structures/04042/index.cfm#toc

Physical Dimension

Physical Dimension

Physical Dimension

Physical Dimension

Reporting: Basic Pin Information

Reporting: Scanning Report – Top of Pin

Reporting: Scanning Report – Bottom of Pin

Mock-Up

Mock-Up

Mock-Up

Mock-Up

Mock-Up

Reporting: Basic Pin Information

Hanger Pin Testing using Shear Wave

Pitch and Catch Methods- Echo Dynamic

Pitch and Catch Methods- Set-up

Pitch and Catch Methods- Echo Dynamic

6.7: Material PropertiesElastic Modulus Measurements

6.7.1

Determination of Microstructural Differences

Ultrasonic methods can be used to determine microstructural differences in metals. For this, contact testing with the pulse-echo technique is used. The testing can be either the measurement of (1) ultrasonic attenuation or the (2) measurement of bulk sound velocity.

6.7.2

The attenuation method

The attenuation method is based on the decay of multiple echoes from test piece surfaces. Once a standard is established, other test pieces can be compared to it by comparing the decay of these echoes to an exponential curve. This test is especially suited for the microstructural control of production parts, in which all that is necessary is to determine whether or not the parts conform to a standard. An example of the use of ultrasonic attenuation in the determination of differences in microstructure is the control of graphite-flake size in gray iron castings, which in turn controls tensile strength. In one application, a water-column search unit that produced a pulsed beam with a frequency of 2.25 MHz was used to test each casting across an area of the casting wall having uniform thickness and parallel front and back surfaces.

A test program had been first carried out to determine the maximum size of graphite flakes that could be permitted in the casting and still maintain a minimum tensile strength of 200 MPa (30 ksi). Then, ultrasonic tests were made on sample castings to determine to what intensity level the second back reflection was lowered by the attenuation effects of graphite flakes larger than permitted. Next, a gate was set on the ultrasonic instrument in the region of the second back reflection, and an alarm was set to signal whenever the intensity of this reflection was below the allowable level. The testing equipment was then integrated into an automatic loading conveyor, where the castings were 100% inspected and passed or rejected before any machining operation.

6.7.3

Velocity Measurements

Velocity Measurements When considering the compressional and shear wave velocities given in Table 1, there may be small deviations for crystalline materials because of elastic anisotropy. This is important and particularly evident in copper, brass, and austenitic steels. The following example illustrates the variation of sound velocity with changes in the microstructure of leaded free-cutting brass.

6.7.4

Elastic Modulus Measurement

Application: Measurement on Young's Modulus and Shear Modulus of Elasticity, and Poisson's ratio, in non-dispersive isotropic engineering materials. Background: 1. Young's Modulus of Elasticity is defined as the ratio of stress (force per unit area) to corresponding strain (deformation) in a material under tension or compression. 2. Shear Modulus of Elasticity is similar to the ratio of stress to strain in a material subjected to shear stress. 3. Poisson's Ratio is the ratio of transverse strain to corresponding axial strain on a material stressed along one axis. http://www.olympus-ims.com/en/applications/elastic-modulus-measurement/ http://www.olympus-ims.com/en/applications/?347[search][sCategoryId][1166017122]=1166017163&347[search][submit]=Search

Elastic Modulus Measurement – Young’s Modulus & Shear Modulus

http://en.wikipedia.org/wiki/Shear_modulus

Elastic Modulus Measurement- Poisson Ratio

These basic material properties, which are of interest in many manufacturing and research applications, can be determined through computations based on measured sound velocities and material density. Sound velocity can be easily measured using ultrasonic pulse-echo techniques with appropriate equipment. The general procedure outlined below is valid for any (1) homogeneous, (2) isotropic, (3) non-dispersive material (velocity does not change with frequency). This includes most common metals, industrial ceramics, and glasses as long as cross sectional dimensions are not close to the test frequency wavelength. Rigid plastics such as polystyrene and acrylic can also be measured, although they are more challenging due to higher sound attenuation. Keyword: non-dispersive material (velocity does not change with frequency).

Rubber cannot be characterized ultrasonically because of its high dispersion and nonlinear elastic properties. Soft plastics similarly exhibit very high attenuation in shear mode and as a practical matter usually cannot be tested. In the case of anisotropic materials, elastic properties vary with direction, and so do longitudinal and/or shear wave sound velocity. Generation of a full matrix of elastic moduli in anisotropic specimens typically requires six different sets of ultrasonic measurements. Porosity or coarse granularity in a material can affect the accuracy of ultrasonic modulus measurement since these conditions can cause variations in sound velocity based on grain size and orientation or porosity size and distribution, independent of material elasticity. Keyword: anisotropic materials, elastic properties vary with direction

Equipment: The velocity measurements for modulus calculation are most commonly made with precision thickness gages such as models 38DL PLUS and 45MG with Single Element software, or a flaw detector with velocity measurement capability such as the EPOCH series instruments. Pulser/receivers such as the Model 5072PR or 5077PR can also be used with an oscilloscope or waveform digitizer for transit time measurements. This test also requires two transducers appropriate to the material being tested, for pulse-echo sound velocity measurement in longitudinal and shear modes. Commonly used transducers include an M112 or V112 broadband longitudinal wave transducer (10 MHz) and a V156 normal incidence shear wave transducer (5 MHz). These work well for many common metal and fired ceramic samples. Different transducers will be required for very thick, very thin, or highly attenuating samples. Some cases may also require use of through transmission techniques, with pairs of transducers positioned on opposite sides of the part. It is recommended that in all cases the user consult Olympus for specific transducer recommendations and assistance with instrument setup.

The test sample may be of any geometry that permits clean pulse/echo measurement of sound transit time through a section on thickness. Ideally this would be a sample at least 0.5 in. (12.5 mm) thick, with smooth parallel surfaces and a width or diameter greater than the diameter of the transducer being used. Caution must be used when testing narrow specimens due to possible edge effects that can affect measured pulse transit time. Resolution will be limited when very thin samples are used due to the small changes in pulse transit time across short sound paths. For that reason we recommend that samples should be at least 0.2 in. (5 mm) thick, preferably thicker. In all cases the thickness of the test sample must be precisely known. Keywords: 1. Caution must be used when testing narrow specimens due to possible edge effects that can affect measured pulse transit time. 2. Resolution will be limited when very thin samples are used due to the small changes in pulse transit time across short sound paths.

Testing Procedure: Equipment Used. Measure the (1) longitudinal and (2) shear wave sound velocity of the test piece using the appropriate transducers and instrument setup. The shear wave measurement will require use of a specialized high viscosity couplant such as our SWC. A Model 38DL PLUS a 45MG thickness gage can provide a direct readout of material velocity based on an entered sample thickness, and an EPOCH series flaw detector can measure velocity through a velocity calibration procedure. In either case, follow the recommended procedure for velocity measurement as described in the instrument's operating manual. If using a pulser/receiver, simply record the round-trip transit time through an area of known thickness with both longitudinal and shear wave transducers, and compute: Question: For measurement of shear wave velocity is normal incident transverse wave used? (hint by the used of highly viscous couplant requirement)

Testing Procedure: Velocity Measurements & Calculations Velocity= Distance / ( ½ Round trip traverse time) Convert units as necessary to obtain velocities expressed as inches per second or centimeters per second. (Time will usually have been measured in microseconds, so multiply in/uS or cm/uS by 106 to obtain in/S or cm/S.) The velocities thus obtained may be inserted into the following equations.

Poisson Ratio (v)

=

Young’s Modulus

=

Shear Modulus

=

Velocity & Equations

Poisson Ratio (v)

=

Young’s Modulus (E)

=

Shear Modulus (G)

=

,

VL, VS = Longitudinal and Shear Velocity v = Poisson ratio p = Material density

Note on units: If sound velocity is expressed in cm/S and density in g/cm3, then Young's modulus will be expressed in units of dynes/cm2. If English units of in/S and lbs/in3 are used to compute modulus in pounds per square inch (PSI), remember the distinction between "pound" as a unit of force versus a unit of mass. Since modulus is expressed as a force per unit area, when calculating in English units it is necessary to multiply the solution of the above equation by a mass/force conversion constant of (1 / Acceleration of Gravity) to obtain modulus in PSI. Alternately, if the initial calculation is done in metric units, use the conversion factor 1 psi = 6.89 x 104 dynes/cm2. Another alternative is to enter velocity in in/S, density in g/cm 3, and divide by a conversion constant of 1.07 x 104 to obtain modulus in PSI.

6.8: High Temperature Ultrasonic Testing

Experts at work

1.0

Background:

Although most ultrasonic flaw detection and thickness gauging is performed at normal environmental temperatures, there are many situations where it is necessary to test a material that is hot. This most commonly happens in process industries, where hot metal pipes or tanks must be tested without shutting them down for cooling, but also includes manufacturing situations involving hot materials, such as extruded plastic pipe or thermally molded plastic immediately after fabrication, or testing of metal ingots or castings before they have fully cooled. Conventional ultrasonic transducers will tolerate temperatures up to approximately 50° C or 125° F. At higher temperatures, they will eventually suffer permanent damage due to internal disbonding caused by thermal expansion. If the material being tested is hotter than approximately 50° C or 125° F, then high temperature transducers and special test techniques should be employed.

http://www.olympus-ims.com/en/applications/high-temperature-ultrasonic-testing/

This application note contains quick reference information regarding selection of high temperature transducers and couplants, and important factors regarding their use. It covers conventional ultrasonic testing of materials at temperatures up to approximately 500°C or 1000°F. In research applications involving temperatures higher than that, highly specialized waveguide techniques are used. They fall outside the scope of this note. Testing Methods used: Methods used to increase the useful range for high temperature application are: ■ Delay Line ■ High temperature Couplants ■ Testing Techniques & Equipment Requirements

Temperature Limitation: Conventional ultrasonic transducers 50°C

Temperature Limitation: Conventional ultrasonic transducers 50°C

Temperature Limitation: Conventional ultrasonic transducers 50°C

http://amazingunseentravel.blogspot.com/2011_08_28_archive.html

Temperature Limitation: Conventional ultrasonic transducers 50°C

Temperature Limitation: Conventional ultrasonic transducers 50°C

http://www.wisdompetals.com/index.php/photos/138-wonder-of-the-world-crescent-lake-in-gopi-deser

Temperature Limitation: Conventional ultrasonic transducers 50°C

http://www.wisdompetals.com/index.php/photos/138-wonder-of-the-world-crescent-lake-in-gopi-deser

敦煌大漠美食- 50度火锅双塔鱼

http://www.cc6uu.com/science/article/raiders/2407

High Temperature Conventional UTGood Till & No-More.

2.0 Methods used for H.Temperature Scanning 2.1 Transducers- H.Temperature Delay Line Material Panametrics-NDT high temperature transducers fall into two categories, ■ dual element transducers and ■ delay line transducers. In both cases, the delay line material (which is internal in the case of duals) serves as thermal insulation between the active transducer element and the hot test surface. For design reasons, there are no high temperature contact or immersion transducers in the standard product line. High temperature duals and delay line transducers are available for both thickness gaging and flaw detection applications. As with all ultrasonic tests, the best transducer for a given application will be determined by specific test requirements, including the material, the thickness range, the temperature, and in the case of flaw detection, the type and size of the relevant flaws.

(1a) Thickness gaging The most common application for high temperature thickness gaging is corrosion survey work, the measurement of remaining metal thickness of hot pipes and tanks with corrosion gages such as Models 38DL PLUS and 45MG. Most of the transducers that are designed for use with Olympus corrosion gages are suitable for high temperature use. The commonly used D790 series transducers can be used on surfaces as hot as 500° C or 930° F. For a complete list of available corrosion gauging duals that includes temperature specifications, see this link: Corrosion Gage Duals.

For precision thickness gauging applications using the Models 38DL PLUS or Model 45MG with Single Element software ,such as hot plastics, any of the standard Micro-scan delay line transducers in the M200 series (including gage default transducers M202, M206, M207, and M208) can be equipped with high temperature delay lines. DLHT-1, -2, and -3 delay lines may be used on surfaces up to 260° C or 500° F. DLHT-101, -201, and -301 delay lines may be used on surfaces up to 175° C or 350° F. These delay lines are listed in the Delay Line Option Chart.

In challenging applications requiring low frequency transducers for increased penetration, the Videoscan Replaceable Face Transducers and appropriate high temperature delay lines can also be used with 38DL PLUS and 45MG thickness gages incorporating the HP (high penetration) software option. Custom transducer setups will be required. Standard delay lines for this family of transducers can be used in contact with surfaces as hot as 480° C or 900° F. For a full list of transducers and delay lines, see this link: Replaceable Face Transducers.

(1b) Flaw detection As in high temperature thickness gaging applications, high temperature flaw detection most commonly uses dual element or delay line transducers. All standard Panametrics-NDT flaw detection duals offer high temperature capability. Fingertip, Flush Case, and Extended Range duals whose frequency is 5 MHz or below may be used up to approximately 425° C or 800° F, and higher frequency duals (7.5 and 10 MHz) may be used up to approximately 175° C or 350° F. For a full list of transducers in this category, see this link: Flaw Detection Duals. All of the Videoscan Replaceable Face Transducers can be used with appropriate high temperature delay lines in flaw detection applications. The available delay lines for this family of transducers can be used in contact with surfaces as hot as 480° C or 900° F. For a full list of transducers and delay lines suitable for various maximum temperatures, see this link: Replaceable Face Transducers.

Applications involving thin materials are often best handled by the delay line transducers in the V200 series (most commonly the V202, V206, V207, and V208), any of which can be equipped with high temperature delay lines. DLHT-1, -2, and -3 delay lines may be used on surfaces up to 260° C or 500° F. DLHT-101, -201, and -301 delay lines may be used on surfaces up to 175° C or 350° F. These transducers and delay lines are listed on the Delay Line Transducer List. We also offers special high temperature wedges for use with angle beam transducers, the ABWHT series for use up to 260° C or 500° F and the ABWVHT series for use up to 480° C or 900° F. Detailed information on available sizes is available from the Sales Department.

2.2 High Temperature Couplants Most common ultrasonic couplants such as propylene glycol, glycerin, and ultrasonic gels will quickly vaporize if used on surfaces hotter than approximately 100° C or 200° F. Thus, ultrasonic testing at high temperatures requires specially formulated couplants that will remain in a stable liquid or paste form without boiling off, burning, or releasing toxic fumes. It is important to be aware of the specified temperature range for their use, and use them only within that range. Poor acoustic performance and/or safety hazards may result from using high temperature couplants beyond their intended range. At very high temperatures, even specialized high temperature couplants must be used quickly since they will tend to dry out or solidify and no longer transmit ultrasonic energy. Dried couplant residue should be removed from the test surface and the transducer before the next measurement.

Note that normal incidence shear wave coupling is generally not possible at elevated temperatures because commercial shear wave couplants will liquify and lose the very high viscosity that is necessary for transmission of shear waves. We offer two types of high temperature couplant: ■ Couplant E - Ultratherm Recommended for use between 500° and 970° F (260° to 520° C) ■ Couplant G - Medium Temperature Couplant Recommended for use at temperatures up to 600° F (315° C). For a complete list of couplants available from Olympus, along with further notes on each, please refer to the application note on Ultrasonic Couplants.

Keyword: Note that normal incidence shear wave coupling is generally not possible at elevated temperatures because commercial shear wave couplants will liquify and lose the very high viscosity that is necessary for transmission of shear waves.

http://www.olympus-ims.com/en/applications/normal-incidence-shear-wave-transducers/ http://static5.olympus-ims.com/data/Flash/shear_wave.swf?rev=3970 http://www.olympus-ims.com/en/ultrasonic-transducers/shear-wave/

2.3 Test Techniques The following factors should always be taken into consideration in establishing a test procedure for any high temperature application: Transducer Time of Contacts All standard high temperature transducers are designed with a duty cycle in mind. Although the delay line insulates the interior of the transducer, lengthy contact with very hot surfaces will cause significant heat buildup, and eventually permanent damage to the transducer if the interior temperature becomes hot enough. For most dual element and delay line transducers, the recommended duty cycle for surface temperatures between approximately 90° C and 425° C (200° F to 800° F) is no more than ten seconds of contact with the hot surface (five seconds is recomended), followed by a minimum of one minute of air cooling. Note that this is guideline only; the ratio of contact time to cooling time becomes more critical at the upper end of a given transducer's specified temperature range.

As a general rule, if the outer case of the transducer becomes too hot to comfortably hold with bare fingers, then the interior temperature of the transducer is reaching a potentially damaging temperature and the transducer must be allowed to cool down before testing continues. Some users have employed water cooling to accelerate the cooling process, however Olympus publishes no official guidelines for water cooling and its appropriateness must be determined by the individual user Keyword: ■ 10 second contact follows by 60 second air cooling ■ Water cooling is not guarantee by Olympus NDT

Coupling Technique: The combination of transducer duty cycle requirements and the tendency of couplants to solidify or boil off at the upper end of their usable thickness range requires quick work on the part of the operator. Many users have found the best technique to be to apply a drop of couplant to the face of the transducer and then press the transducer firmly to the test surface, without twisting or grinding it (which can cause transducer wear). Any dried couplant residue should be removed from the transducer tip between measurements.

2.4 Equipment Functions Freeze Function Olympus Epoch series flaw detectors and all thickness gages have freeze functions that can be used to freeze the displayed waveform and reading. The freeze function is very useful in high temperature measurements because it allows the operator to capture a reading and quickly remove the transducer from the hot surface. With gages, the fast screen update mode should be used to help minimize contact time. High Gain Boost Gain Boost: The 38DL PLUS and 45MG gages have user adjustable gain boost functions, as do all Epoch series flaw detectors. Because of the higher attenuation levels associated with high temperature measurements, it is often useful to increase gain before making measurements.

3.0

High Temperature Testing and Variability

3.1

Velocity Variation:

Sound velocity in all materials changes with temperature, slowing down as the material heats up. Accurate thickness gaging of hot materials always requires velocity recalibration. In steel, this velocity change is approximately 1% per 55°C or 100°F change in temperature. (The exact value varies depending on the alloy.) In plastics and other polymers, this change is much greater, and can approach 50% per 55°C or 100°F change in temperature up to the melting point. If a temperature/velocity plot for the material is not available, then a velocity calibration should be performed on a sample of the test material at the actual test temperature. The temperature compensation software function in the 38DL PLUS gage can be used to automatically adjust velocity for known elevated temperatures based on a programmed temperature/velocity constant. Keyword: ■ Velocity change of -1% (minus) per 55°C or 100°F change in temperature ■ Temperature versus velocity plot

Keyword: ■ Velocity change of -1% (minus) per 55°C or 100°F change in temperature ■ Temperature versus velocity plot

3.2

Zero Recalibration:

When performing thickness gaging with dual element transducers, remember that the zero offset value for a given transducer will change as it heats up due to changes in transit time through the delay line. Thus, periodic re-zeroing is necessary to maintain measurement accuracy. With Olympus corrosion gages this can be quickly and easily done through the gage's auto-zero function; simply press the 2nd Function > DO ZERO keys.

3.3

Increased Attenuation:

Sound attenuation in all materials increases with temperature, and the effect is much more pronounced in plastics than in metals or ceramics. In typical fine grain carbon steel alloys, attenuation at 5 MHz at room temperature is approximately 2 dB per 100 mm one-way sound path (equivalent to a round trip path of 50 mm each way). At 500°C or 930°C, attenuation increases to approximately 15 dB per 100 mm of sound path. This effect can require use of significantly increased instrument gain when testing over long sound paths at high temperature, and can also require adjustment to distance/amplitude correction (DAC) curves or TVG (Time Varied Gain) programs that were established at room temperature. Temperature/attenuation effects in polymers are highly material dependent, but will be typically be several times greater than the above numbers for steel. In particular, long high temperature delay lines that have heated up may represent a significant source of total attenuation in a test.

Keyword:  In typical fine grain carbon steel alloys, attenuation at 5 MHz at room temperature is approximately 2 dB per 100 mm one-way sound path (equivalent to a round trip path of 50 mm each way).  At 500°C or 930°C, attenuation increases to approximately 15 dB per 100 mm of sound path.

3.4

Angular Variation in Wedges:

With any high temperature wedge, sound velocity in the wedge material will decrease as it heats up, and thus the refracted angle in metals will increase as the wedge heats up. If this is of concern in a given test, refracted angle should be verified at actual operating temperature. As a practical matter, thermal variations during testing will often make precise determination of the actual refracted angle difficult. Keyword: As a practical matter, thermal variations during testing will often make precise determination of the actual refracted angle difficult.

Discussion: An offshore installation of Topside to Jacket Legs, hot conventional Ultrasonic Testing at elevated temperature below 500 C was proposed. What are the critical information to be reviewed? Hints: High temperature testing methods used & limitations Variability due to high temperature & concerns

6.9: Dimension-Measurement Applications

6.9.1

Dimension-Measurement Applications

Ultrasonic inspection methods can be used for measurement of metal thickness. These same methods can also be used to monitor the deterioration of a surface and subsequent thinning of a part due to wear or corrosion and to determine the position of a solid object or liquid material in a closed metallic cavity.

6.9.2

Thickness measurements

are made using pulse-echo techniques. Resonance techniques were also used in the past, but have become obsolete. The results can be read on an oscilloscope screen or on a meter, or they can be printed out. Also, the same data signals can be fed through gates to operate sorting or marking devices or to sound alarms. Resonance thickness testing was most often applied to process control inspection where opposite sides of the test pieces are smooth and parallel, such as in the inspection of hollow extrusions, drawn tubes, tube bends, flat sheet and plate, or electroplated parts. The maximum frequency that can be used for the test determines the minimum thickness that can be measured. The maximum thickness that can be measured depends on such test conditions as couplant characteristics, test frequency, and instrument design and on material type, metallurgical condition, and surface roughness.

Pulse-echo thickness gages with a digital readout are widely used for thickness measurement. Pulse-echo testing can measure such great thickness that it can determine the length of a steel reinforcing rod in a concrete structure, provided one end of the rod is accessible for contact by the search unit. Although pulse-echo testing is capable of measuring considerable thicknesses, near-field effects make the use of pulse-echo testing ineffective on very thin materials.

6.9.3

Position measurements

Position measurements of solid parts or liquid materials in closed metallic cavities are usually made with pulse echo type equipment. One technique is to look for changes in back reflection intensity as the position of the search unit is changed. In one variation of this technique, the oil level in differential housings was checked to see if the automated equipment used to put the oil in the housing on an-assembly line had malfunctioned. The test developed for this application utilized a dual-gated pulse-echo system that employed a 1.6MHz immersion-type search unit with a thin, oil filled rubber gland over its face. The search unit was automatically placed against the outside surface of the housing just below the proper oil level, as shown in Fig. 60(a).

With oil at the correct level, sufficient beam energy was transmitted across the boundary between the housing wall and the oil to attenuate the reflected beam so that multiple back reflections were all contained in the first gate (Fig. 60b). The lack of oil at the correct level allowed the multiple back reflections to spill over into the second gate (Fig. 60c). Thus, the test was a fail-safe test that signaled "no test" (no signal in the first gate), "go" (signals in the first gate only), and "no go" (signals in both gates).

Fig. 60 Method of determining correct oil level in on automobile differential housing by use of an ultrasonic pulse-echo system. See text.

In another position measurement system, a set of two contact-type 4-MHz search units was utilized in a through transmission pitch-catch arrangement to determine the movement of a piston in a hydraulic oil accumulator as both precharge nitrogen-gas pressure and standby oil pressure varied (Fig. 61). The two search units were placed 180° apart on the outside surface of the accumulator wall at a position on the oil side of the piston, as shown in Fig. 61. When a high energy pulse was sent from the transmitting unit, the beam was able to travel straight through the oil, and a strong signal was picked up by the receiving unit. However, as the search units were moved toward the piston (see locations drawn in phantom in Fig. 61), the sloping sides of the recess in the piston bottom deflected the beam so that very little signal was detected by the receiving unit.

Fig. 61 Setup for determining the position of a piston in a hydraulic oil accumulator by use of two contact search units utilizing a through transmission arrangement

Q144. A thin sheet may be inspected with the ultrasonic wavw direction normal to the surface by observing: A. B. C. D.

The amplitude of the front surface reflection The multiple reflection pattern All front surface reflection None of the above

6.10: In-Service Inspection

In-Service Inspection The methods described above are applied in the course of and immediately after the production process and are therefore called production tests. To survey highly stressed parts, especially in power plants, repeated tests or inservice inspections are becoming more and more important. In these inspections any defects identified earlier but not being a cause for rejection can be observed for any changes caused by the service conditions. In addition service-produced defects must be detected, these being mainly cracks caused by thermal shock, fatigue or creep, or by corrosion attack.

In-Service Inspection- Testing for fatigue cracks on crankshafts and crankpins. a Without bore; b with bore

In-Service Inspection- Oblique or skewed fatigue cracks on crankpins

In-Service Inspection- (a) Crack test on press columns, pump rods, etc. (b) Crack test on thread in the shadow of a sound beam; schematic screen picture above

In-Service Inspection- (a) Probe for detecting fatigue cracks in turbine discs (design Krautkriimer-Branson) (b) Detection of cracks in riveted turbine blades

In-Service Inspection- (a) Testing methods for conical defects in a bolt (b) Testing for fatigue cracks in bolts

In-Service Inspection- (a) Cross-section through a leaf spring for railway cars with quenching crack showing testing with small angle probe or normal probe. The use of surface waves is unfavorable due to roughness (b)Testing a helical spring for quenching cracks, using surface waves

6.11: Casting

Casting In castings flaw detection is almost exclusively concerned with manufacturing defects and only rarely as in-service inspection. Suitable testing techniques and the subsequent evaluation of indications in castings is very different from the testing of forged and worked material so that the differences must not be forgotten or difficulties can occur. In-service inspection, as in the case of forgings, depends on the local stresses and the piece geometry so it is not necessary to treat it specially in this section.

Casting- Typical casting defects and their detection methods

Casting

Casting- Detection of shrinkage cavities with normal and angle probes

6.12: Bonded Joint

Inspection of Bonded Joints If the shape of a joint is favorable, ultrasonic inspection can be used to determine the soundness of joints bonded either adhesively or by any of the various metallurgical methods, including brazing and soldering. Both pulseecho and resonance techniques have been used to evaluate bond quality in brazed joints. A babbitted sleeve bearing is a typical part having a metallurgical bond that is ultrasonically inspected for flaws. The bond between babbitt and backing shell is inspected with a straight-beam pulse-echo technique, using a contacttype search unit applied to the outside of the steel shell. A small-diameter search unit is used to ensure adequate contact with the shell through the couplant. Before inspection, the outside of the steel shell and the inside of the cast babbitt liner are machined to a maximum surface roughness of 3.20 μm (125 μ in.) (but the liner is not machined to final thickness).

During inspection, the oscilloscope screen normally shows three indications: the initial pulse, a small echo from the bond line (due to differences in acoustical impedance of steel and babbitt), and the back reflection from the inside surface of the liner. Regions where the bond line indication is minimum are assumed to have an acceptable bond. Where the bond line signal increases, the bond is questionable. Where there is no back reflection at all from the inside surface of the liner (babbitt), there is no bond. Inspection of other types of bonded joints is often done in a manner similar to that described above for babbitted bearings. An extensive discussion of the ultrasonic inspection of various types of adhesive-bonded joints (including two-component lap joints, three component sandwich structures, and multiple-component laminated structures) is available in the article "AdhesiveBonded Joints" in this Volume.

6.13: Corrosion Monitoring

Corrosion Monitoring Ultrasonic inspection can be used for the in situ monitoring of corrosion by measuring the thickness of vessel walls with ultrasonic thickness gages. The advantage of this method is that internal corrosion of a vessel can be monitored without penetration. There are, however, some disadvantages. Serious problems may exist in equipment that has a metallurgically bonded internal lining, because it is not obvious from which surface the returning signal will originate. A poor surface finish, paint, or a vessel at high or low temperature may also complicate the use of contact piezoelectric transducers (although this difficulty might be addressed by noncontact in situ inspection with an EMA transducer).

Despite these drawbacks, ultrasonic thickness measurements are widely used to determine corrosion rates. To obtain a corrosion rate, a series of thickness measurements is made over an interval of time, and the metal loss per unit time is determined from the measurement samples. Hand-held ultrasonic thickness gages are suitable for these measurements and are relatively easy to use. However, depending on the type of transducer used, the ultrasonic thickness method can overestimate metal thicknesses when the remaining thickness is under approximately 1.3 mm (0.05 in.). Another corrosion inspection method consists of monitoring back-surface roughness with ultrasonic techniques. The following example describes an application of this method in the monitoring of nuclear waste containers.

6.14: Crack Monitoring

Crack Monitoring Laboratory and in-service monitoring of the initiation and propagation of cracks that are relatively slow growing (such as fatigue cracks, stress-rupture cracks, and stress-corrosion cracks) has been accomplished with ultrasonic techniques. An example of the ultrasonic detection of stress-rupture cracks resulting from creep in reformer-furnace headers is given in the article "Boilers and Pressure Vessels" in this Volume. A relatively new and improved approach for monitoring the growth of cracks is done with ultrasonic imaging techniques.

Monitoring of fatigue cracks in parts during laboratory tests and while in service in the field has been extensively done using ultrasonic techniques. Reference 13 describes the use of surface waves to detect the initiation of cracks in cylindrical compression-fatigue test pieces having a circumferential notch. The surface waves, which were produced by four angle-beam search units on the circumference of each test piece, were able to follow the contour of the notch and detect the cracks at the notch root. Monitoring the crack-growth rate was accomplished by periodically removing the cracked test piece from the stressing rig and measuring the crack size by straight-beam, pulse-echo immersion inspection. It was found necessary to break open some of the cracked test pieces (using impact at low temperature) and visually measure the crack to establish an accurate calibration curve of indication height versus crack size.

The use of pulse-echo techniques for monitoring fatigue cracks in pressure vessels in laboratory tests is described in Ref 14. These techniques use several overlapping angle-beam (shear wave) search units, which are glued in place to ensure reproducible results as fatigue testing proceeded. The inservice monitoring of fatigue cracking of machine components is often accomplished without removing the component from its assembly.

For example, 150 mm (6 in.) diam, 8100 mm (320 in.) long shafts used in pressure rolls in papermaking machinery developed fatigue cracks in their 500 mm (20 in.) long threaded end sections after long and severe service. These cracks were detected and measured at 3-month intervals, using a contact-type straight-beam search unit placed on the end of each shaft, without removing the shaft from the machine. When the cracks were found to cover over 25% of the cross section of a shaft, the shaft was removed and replaced. In another case, fatigue cracking in a weld joining components of the shell of a ball mill 4.3 m (14 ft) in diameter by 9.1 m (30 ft) long was monitored using contact type angle-beam search units. The testing was done at 3-month intervals until a crack was detected; then it was monitored more frequently. When a crack reached a length of 150 mm (6 in.), milling was halted and the crack repaired.

6.15: Stress Measurements

Stress Measurements With ultrasonic techniques, the velocity of ultrasonic waves in materials can be measured and related to stress (Ref 16). These techniques rely on the small velocity changes caused by the presence of stress, which is known as an acousto-elastic effect. The technique is difficult to apply because of the very small changes in velocity with changes in stress and because of the difficulty in distinguishing stress effects from material variations (such as texture; see Ref 17). However, with the increased ability to time the arrival of ultrasonic pulses accurately (±1 ns), the technique has become feasible for a few practical applications, such as the measurement of axial loads in steel bolts and the measurement of residual stress (Ref 5). .

The real limitation of this technique is that in many materials the ultrasonic pulse becomes distorted, which can reduce the accuracy of the measurement. One way to avoid this problem is to measure the phase difference between two-tone bursts by changing the frequency to keep the phase difference constant (Ref 5). Small specimens are used in a water bath, and the pulses received from the front and back surfaces overlap. The presence of stress also rotates the plane of polarization of polarized shear waves, and there is some correlation between the angle of rotation and the magnitude of the stress. Measurement of this rotation can be used to measure the internal stress averaged over the volume of material traversed by the ultrasonic beam.

6.16: Bond Testing

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6.App-1: TOFD Introduction NOTE: Not in the exam syllabus or BOK

6.App-1.1

TOFD Basic Theory

TOFD is usually performed using longitudinal waves as the primary detection method. Ultrasonic sensors are placed on each side of the weld. One sensor sends the ultrasonic beam into the material and the other sensor receives reflected and diffracted ultrasound from anomalies and geometric reflectors.

TOFD provides a wide area of coverage with a single beam by exploiting ultrasonic beam spread theory inside the wedge and the inspected material. When the beam comes in contact with the tip of a flaw, or crack, diffracted energy is cast in all directions. Measuring the time of flight of the diffracted beams enables accurate and reliable flaw detection and sizing, even if the crack is off-oriented to the initial beam direction. During typical TOFD inspections, A-scans are collected and used to create Bscan (side view) images of the weld. Analysis is done on the acquisition unit or in post-analysis software, positioning cursors to measure the length and through-wall height of flaws. Keywords: ■ ■ ■ ■ ■

Tip Diffraction Off-oriented to the initial beam direction Time of Flight A-scan / B-scan Post analysis software

6.App-1.2

Main Benefits of TOFD for Weld Inspection

 Based on diffraction, so relatively indifferent to weld bevel angles and flaw orientation  Uses time of arrival of signals received from crack tips for accurate defect positioning and sizing  Precise sizing capability makes it an ideal flaw monitoring method  Quick to set up and perform an inspection, as a single beam offers a large area of coverage  Rapid scanning with imaging and full data recording  Can also be used for corrosion inspections  Required equipment is more economical than phased array, due to conventional nature (single pulser and receiver) and use of conventional probes  Highly sensitive to all weld flaw types

TOFD offers rapid weld inspection with excellent flaw detection and sizing capacities. The diffraction technique provides critical sizing capability with relative indifference to bevel angle or flaw orientation. TOFD can be utilized on its own or in conjunction with other NDT techniques.

6.App-1.3

More Reading on Time of Flight Diffraction (TOFD)

6.App-1.3.1 The Theory Time of flight diffraction (TOFD) detects flaws using the signals diffracted from the flaw’s extremities. Two angled compression wave probes are used in transmit-receive mode, one each side of the weld. The beam divergence is such that the majority of the thickness is inspected, although, for thicker components, more than one probe separation may be required. When the sound strikes the tip of a crack, this acts as a secondary emitter which scatters sound out in all directions, some in the direction of the receiving probe. A ‘lateral wave’ travelling at the same velocity as the compression waves, travels directly from the transmitter to the receiver. The time difference between the lateral wave and the diffracted signal from the flaw provides a measure of its distance from the scanned surface. If the flaw is large enough in the through wall dimension, it may be possible to resolve the tip diffracted signals from its top and bottom, thereby allowing the through wall height of the flaw to be measured. http://www.iteglobal.com/services/advanced-ndt/time-of-flight-diffraction-tofd/

Due to the low amplitude of the diffracted signals, TOFD is usually carried out using a preamplifier and hardware designed to improve signal-to-noise performance. As the probes are scanned along the weld, the RF A-Scan signals are digitised and displayed in the form of a grey-scale image showing flaws as alternating white and black fringes. Depending on which direction the probes are moved over the component surface, it is possible to construct ‘end-view’; (B-scan TOFD) or ‘side-view’ (D-scan TOFD) cross-sectional slices. TOFD can also utilise Synthetic Aperture focusing or beam modelling software to minimise the effects of beam divergence, thereby providing more accurate location and sizing information.

TOFD is generally recognised as the most accurate ultrasonic technique for measuring the through-wall height of planar flaws that lie perpendicular to the surface and as a method for detecting and quantifying crevice corrosion at the weld root. At present, national standards for the application of TOFD exist, however, no acceptance criteria have been agreed upon. The TOFD technique is suited for the detection and sizing of all types of embedded flaws, especially those planar in nature. However, the detection of small near the scan surface flaws can be more difficult due to the presence of the lateral wave response which often occupies several millimeters of the depth axis on images.

Tips Diffractions

TOFD Transmitter

Receiver Diffracted wave from upper end of crack Diffracted wave from lower end of crack Crack Back-wall echo Crack height can be calculate by measuring propagation delayed time of diffraction wave

Diffracted wave from upper end of crack

Lateral wave Diffracted wave from lower end of crack

TOFD

6.App-1.2 Application Examples ■ TOFD for Weld Root Corrosion and Erosion For piping and other flow systems, certain conditions exist that lead to corrosion and erosion in the weld root and the heat-affected zone (HAZ) of the weld. The contributing factors are often metallurgical, chemical, or flow related, and the resulting metal loss can lead to failure of the weld/base metal. The shape of the corroded or eroded weld or base metal can make ultrasonic inspection extremely difficult to apply, thus impeding accurate detection and measurement of anomalies. The time-of-flight diffraction (TOFD) technique proves to be a valid option for evaluating weld root corrosion and erosion, as well as similar conditions such as FAC (flow-accelerated corrosion). The goal of any of these inspections is to accurately measure the wall thickness, the weld, and the HAZ. The unpredictable shape of the remaining material often makes pulse-echo ultrasonic inspection ineffective.

http://www.olympus-ims.com/en/applications/tofd-for-weld-root-corrosion-and-erosion/

TOFD has been used for some time for general weld inspections. It has proven to be a rapid and easily deployable method with an excellent capacity for sizing. One of the inherent strengths of TOFD for detection and sizing purposes is its relative indifference to the orientation of defects because of its primary use of diffracted versus reflected energy. The TOFD technique utilizes two transducers: a transmitter transducer floods the inspected region with sound in the forward direction; on the opposite side of the weld, a receiver transducer is positioned to receive diffracted and reflected energy from the back wall or from anomalies present in the region. Common pulse-echo techniques can be misdirected by the shape of the region, resulting in imprecise measurement and assessment.

Figure 5-3 – Preferential weld corrosion in lean amine (Reference 5)

Figure 5-2 – Hot Lean Amine Corrosion of Carbon Steel:

Weld Root Corrosion and Erosion

Pulse-echo shear wave beam being reflected at an off angle.

Illustration of diffracted energy reflecting off weld root/HAZ in all directions.

For these types of weld inspections, TOFD is typically performed from three positions for each weld: (1) centered on the weld, (2) offset to the left, and (3) offset to the right. Scanning from these particular positions helps to achieve the best results. This method ensures detection of the highest point of material loss, determines from which side of the weld the erosion/corrosion indications are originating, and eliminates any masking caused by the back wall signal. Depending on the instrument, these scans can be run concurrently or in separate acquisitions.

TOFD is deployed by scanning the weld with a semiautomatic or fully automatic scanner. Scan settings are set to determine scan resolution. The resulting data file can be saved indefinitely for review and comparison to future scans. After data is acquired, it is analyzed to identify any areas of concern, either directly on the instrument or in post-analysis software. Shifts in data (time/depth) are measured in order to assess the severity of metal loss. The cursors can then be positioned to define areas for depth or thickness measurement readings. Weld defects such as porosity, lack of fusion, and cracking can also be detected when scanning for corrosion and erosion.

Scan of weld with cursor positioned on an uncorroded area; A-scan shows good lateral wave and back wall signal with no indications in between.

Scan of weld with cursor positioned on a corroded area; A-scan shows shift in time of back wall signal from material loss.

Measurement of good area shows thickness as 7.39 mm; TOFD (m-r) reading shows the distance between the positioned cursors.

Measurement of corroded area shows thickness as 5.28 mm; cursors are positioned at top of plate (0) and highest point of material loss. In this example, there is 2.11 mm of material loss due to corrosion.

6.11.3.3TOFD for Corrosion Measurement Equipment (Typical)  OmniScan SX or MX2 (PA or UT models, depending on the number of channels desired and if phased array capability is needed).  TOFD circumferential scanner (HST-Lite or similar, depending on the desired number of probe holders and other application specifics; for example, pipe versus plate).  TOFD probe and wedges (various frequencies, angles, and materials).  Couplant delivery system, WTR-SPRAYER-8L or similar.  TomoView Analysis or OmniPC post-analysis software (optional).

6.App-1.3.4       

TOFD Benefits for Corrosion/Erosion Measurement

Rapid scanning. Cost effective. Auditable and retrievable permanent data sets. Accurate sizing capability. Excellent detection, even on irregularly shaped areas of metal loss. Fast post-acquisition analysis results. Portable and user-friendly TOFD scanning packages.

TOFD for Weld- TOFD Parallel Scanning

6.App-1.3.5

Overview on Scanning Direction

Most typical TOFD inspections are performed with the send and receive transducers on opposite sides of the weld and scanning movement parallel to the weld axis. The main purpose of this “perpendicular” (defined by beam to weld relationship) scanning is to quickly perform weld inspection with the weld cap or re-enforcement in place. This technique can give location in the scan axis, the indication length, height of indication and flaw characterization information. One of the weaknesses of this technique is the lack of index positioning (or where between the probes) the indication is located. This information is usually obtained with complimentary pulse echo ultrasonics when the weld is left in place.

■

Perpendicular Scanning

Scanning direction “parallel” to the weld axis. Beam direction “perpendicular” to the weld axis.

?

Carriage movement direction

One of the weaknesses of this technique is the lack of index positioning (or where between the probes) the indication is located.

■

Parallel TOFD scanning:

Where the scan direction and beam direction are the same is less used, for obvious reasons of not being able to cover the entire length of weld rapidly, more complex movement pattern required of scanner mechanisms, and complexity of the data output of an entire weld inspected. This technique does have advantages when it is possible to be performed.

Typical “Perpendicular” Weld Scanning Setup and Data Collected. Data is side view of weld from scan start to scan finish down the weld. Position of encoder and scanning direction are highlighted.

Typical “Parallel” Weld Scanning Setup and Data Collected. Data is side view of weld from scan start to scan finish across the weld. Position of encoder and scanning direction are highlighted.

■

Benefit of TOFD Parallel Scanning

Although perpendicular TOFD scanning down the weld can give highly accurate depth measurement, generally speaking a parallel scan will give more accurate depth information as well as flaw information, and location in the index position in the weld. With perpendicular scanning, no index position is possible without multiple offset scans being performed or complimentary NDT techniques to position the flaw. In parallel scanning Index position is ascertained by locating the minimum time peak, which corresponds to when the indication is centered between the two probes. For these reasons this technique is often used in critical crack sizing inspections, as well as change monitoring, in other words, monitoring a crack or other defect for growth until it reaches a critical level at which time it is repaired or replaced. For these reasons the technique is often performed on critical components that are costly to shut down for repair, often in the Power Generation industry. More information is often gathered from the flaw as diffraction occurs across the flaw instead of just down the flaw.

6.App-1.3.6 Further Reading- Introduction to Phased Array  http://www.olympus-ims.com/en/ndt-tutorials/intro/ut/

The Experts at work.

Break Time

mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/39440/v0001/reflector:20587?BBCUID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f55674c540f1856834&SSO2-UID

Break Time

mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/39440/v0001/reflector:20587?BBCUID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f55674c540f1856834&SSO2-UID

Break Time

mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/39440/v0001/reflector:20587?BBCUID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f55674c540f1856834&SSO2-UID

Break Time

Sail Off

Section 7: Reference Materials

Content: Section 7: Reference Material 7.1: UT Material Properties 7.2: General References & Resources 7.3: Video Time

7.1: UT Material Properties Acoustic Properties - Piezoelectric Materials Acoustic Properties - Transducers Acoustic Properties - Metals Acoustic Properties - Powdered Metals Acoustic Properties - Liquid Metals Acoustic Properties - Plastics, Resins Acoustic Properties - Rubber Acoustic Properties - Ceramics Acoustic Properties - Wood Acoustic Properties - Liquids Acoustic Properties - Liquid Gases Acoustic Properties - Gases Acoustic Properties - Vapors Acoustic Properties - Body Tissue https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/Reference%20Information/matproperties.htm

7.2: General References & Resources Auld, B.A., Acoustic Fields and Waves in Solids, Vol I & II, 2nd edition Krieger Publishing Company, February 1990; ISBN: 089874783X Cartz, Louis, Nondestructive Testing : Radiography, Ultrasonics, Liquid Penetrant, Magnetic Particle, Eddy Current, ASM Intl; ISBN: 0871705176 Krautkramer, Josef and Krautkramer, Herbert, Ultrasonic Testing of Materials, 4th/revised edition, Springer Verlag, November 1990, ISBN: 0387512314 Diederichs, Rolf and Ginzel, Ed, Nondestructive Testing Encyclopedia, UT Formulae, NDT net http://www.ndt.net/ndtaz/ndtaz.php Ultrasonic Characterization of Materials, NIST, Materials Reliability Division

7.3: Video Time

Calibrating 70° Probe with IIW Block (50%FSH on 1.5mm SDH) to AWS D1.1 (Repeat-Code1)

www.youtube.com/embed/Qr0dGNuq9yY

Section 8: Ultrasonic Inspection Quizzes

Content: Section 8: Ultrasonic Inspection Quizzes 8.1: Ultrasonic Inspection Quizzes 8.2: Online UT Quizzes

8.1: Ultrasonic Inspection Quizzes

http://www-pub.iaea.org/MTCD/publications/PDF/TCS-45_web.pdf

Ultrasonic Inspection Quizzes

Ultrasonic Inspection Quizzes

http://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/mineralsmetals/files/pdf/ndt-end/rad-rad-eng.pdf

8.2: Online UT Quizzes

https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/Quiz/UT%20Quizzes.htm

http://www.ndtcalc.com/index.php?page=quiz&method=ut&qs=10

http://www.studyblue.com/notes/note/n/ut-asnt-level-ii/deck/6278710

Addendum-01a Equipment Calibrations My ASNT Level III UT Study Notes. 2014-June

Normal Beams Calibration Techniques

Attenuation due to Beam spread for:  Large Reflector  Small Reflector

Attenuation Due to Beam Spread: Large Reflector

Attenuation Due to Beam Spread: Small Reflector

SH1 D1

D2 SH2

Material Attenuation Determination:

Material Attenuation Determination: Actual BWE display

IF zero Material attenuation: The second BWE at twice the distance will be exactly 6dB less (50% less), half the 1st BWE height ( ½ FSH). However this is never the case!

Δ dB = total Material attenuation at twice distance travel. Material attenuation =

ΔdB

D1

D2

Material Attenuation in 100mm = YdB-XdB-6dB Material attenuation in dB/mm = (YdB-XdB-6dB)/100 YdB-XdB-6dB X dB

Y dB

Construction of beam edges plot- Normal Transducer

Construction of Beam Edges:

20dB drop to find edges of beam

The other edge:

Construction of beam spread at 13mm:

Construction of beam spread at 25mm:

Construction of beam spread at 32mm:

Angle Beams Calibration Techniques

Perspex as Matching Layer/Wedge

Tunsten impregnated epoxy resin

θs1

2730m/s

θs2

3250m/s

Perspex as Matching Layer/Wedge 1. The Shear wave velocity of Perspex is 2730m/s, the shear wave velocity od steel is 3250m/s. The refracted angle of Perspex ϴS1 is always smaller than ϴS2 2. Pespex is very absortive and attenuated efficiently, thus reflected compressional wavw will be dampen.

First/ Second Critical Angles VL1= 2730m/s, VS2= 3250m/s, VL2= 5900m/s 1st / 2nd critical angle 27.56° 57.14°

Ist Critical angle= 27.56° °

2nd Critical angle= 57.14°

B 33.42°

First/ Second Critical Angles 27.56°

57.14°

°

33.42°

Finding the probe index

Finding the probe index

Checking the probe Angle:

Calibration for range:

Calibration for range:

Angle Beam- Beam edges Proving (Vertical Axis) using IOW Block Stand Off Measurement Techniques.

Stand-off 2 Stand-off 1 Stand off 2

Angle Beam- Beam edges Proving (Vertical Axis) using IOW Block Botoom edge.

Angle Beam- Beam edges Proving (Vertical Axis) using IOW Block Bottom Edge.

The IOW Block: The Institute of Welding Block

The Proofing: Plot out the Stand-Off1 & 2 readings on a transparent slide, superimposed the ploted transparent slide on IOW Block

Angle Beam- Beam edges Proving (Horizontal Axis) using IIW Block

Angle Beam- Beam edges Proving (Horizontal Axis) using IIW Block

Angle Beam- Beam edges Proving (Horizontal Axis) using IIW Block

Scanned at ½ , 1, 1 ½ Skips

Angle Beam- Beam edges Proving (Horizontal Axis) using IIW Block

Angle Beam- Beam edges Proving (Horizontal Axis) using IIW Block

½ Skip 1 Skip 1½ Skip

The DAC

The DAC

DAC Curve

DAC Curve

DAC Curve Plot 1. Obtained the signal from the refernce reflector and mark on the graticule/traspatrent sheet with gain setting at 80% FSH. 2. Set the gain control -6bB and marks the 50% mark. 3. Set the gain contril to the 4. Obtained the signal at the gain setting in item 1 and repeat the process at different sound paths. 5. Plot the curves at the gain setting and -6dB. 6. Determined the transfer correction. 7. Scanned the work pieces at the “Gain Setting + Transfer Correction”

FLAT Bottom Holes FBH

FLAT Bottom Holes FBH

Reading on: FLAT Bottom Holes FBH

https://www.cnde.iastate.edu/ultrasonics/grain-noise

FLAT Bottom Holes FBH A type of reflector commonly used in reference standards. The end (bottom) surface of the hole is the reflector.E quivalent:, the size of a flat-bottom hole which at the same range, gives an ultrasonic indication equal to the one from the discontinuity. This reflector is used in DGS curves, or many calibration blocks, or standards such as the GE specification.

Transfer Corection

Transfer Correction: Reference surface are smooth and scale free unlike the actual work pieces. These call for transfer correction to account for transfer loss resulting from actual scanning.

Transfer Correction: Reference surface are smooth and scale free unlike the actual work pieces. These call for transfer correction to account for transfer loss resulting from actual scanning.

Transfer Correction: Reference surface are smooth and scale free unlike the actual work pieces. These call for transfer correction to account for transfer loss resulting from actual scanning.

Transfer Correction:

Transfer Correction: Comparison of BWE for Compression Probe

Test Material curve

Gain Setting

Reference Block curve

Transfer correction at thickness

Measured point Beam path

Transfer Correction: Compression Probe Method, Plot a curve of gain setting for FSH at different south paths for actual and reference block, the different in gain control at thickness is the transfer correction.

Transfer Correction: Angle Probes Methos, used 2 eaqual angle probes, pitch and catch in the test material ans using the reference block. The differences in gain setting is the transfer correction,

DGS- Distance Gain Size

http://www.sonostarndt.com/EnProductShow.asp?ID=198

FLAT Bottom Holes FBH ■

DGS/AVG

DGS is a sizing technique that relates the amplitude of the echo from a reflector to that of a flat bottom hold at the same depth or distance. This is known as Equivalent Reflector Size or ERS. DGS is an acronym for Distance/Gain/Size and is also known as AVG from its German name, Abstand Verstarkung Grosse. Traditionally this technique involved manually comparing echo amplitudes with printed curves, however contemporary digital flaw detectors can draw the curves following a calibration routine and automatically calculate the ERS of a gated peak. The generated curves are derived from the calculated beam spreading pattern of a given transducer, based on its frequency and element diameter using a single calibration point. Material attenuation and coupling variation in the calibration block and test specimen can be accounted for. http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/dgs-avg/

DGS is a primarily mathematical technique originally based on the ratio of a circular probe’s calculated beam profile and measurable material properties to circular disk reflectors. The technique has since been further applied to square element and even dual element probes, although for the latter, curve sets are empirically derived. It is always up to the user to determine how the resultant DGS calculations relate to actual flaws in real test pieces. An example of a typical DGS curve set is seen below. The uppermost curve (Curve #1) represents the relative amplitude of the echo from a flat plate reflector in decibels, plotted at various distances from the transducer, and the curves below (Curve #2) represent the relative amplitude of echoes from progressively smaller disk reflectors over the same distance scale.

(Curve #1) represents the relative amplitude of the echo from a flat plate reflector in decibels, plotted at various distances from the transducer

(Curve #2) represent the relative amplitude of echoes from progressively smaller disk reflectors over the same distance scale.

As implemented in contemporary digital flaw detectors, DGS curves are typically plotted based on a reference calibration off a known target such as a backwall reflector or a flat bottom hole at a given depth. From that one calibration point, an entire curve set can be drawn based on probe and material characteristics. Rather than plotting the entire curve set, instruments will typically display one curve based on a selected reflector size (registration level) that can be adjusted by the user. In the example below, the upper curve represents the DGS plot for a 2 mm disk reflector at depths from 10 mm to 50 mm. The lower curve is a reference that has been plotted 6 dB lower. In the screen at left, the red gate marks the reflection from a 2 mm diameter flat bottom hole at approximately 20 mm depth. Since this reflector equals the selected registration level, the peak matches the curve at that depth. In the screen at right, a different reflector at a depth of approximately 26 mm has been gated. Based on its height and depth in relation to the curve the instrument calculated an ERS of 1.5 mm.

(Curve #2) represent the relative amplitude of echoes from progressively smaller disk reflectors over the same distance scale.

More reading on DGS

DGS- Different sizes of FBH at different distance

DGS

# of near field

What is DGS TCG is a time-corrected DAC so that equal dimension reflectors give equal amplitude responses for all sound path distances. Used for PAUT Sectorial scans where it would be otherwise impossible to set every angle and sound path to the same sensitivity level using DAC's. ASTM E-1316: DGS (distance gain size-German AVG)distance amplitude curves permitting prediction of reflector size compared to the response from a back surface reflection. The probe manufacturer supplies data sheet diagams for each probe which shows the amplitude response curves from the backwall and a range of diameters of flat-bottom holes along the length of the soundfield. Have a look at EN 583-2:2001 Sensitivity and range setting for excellent authoritative descriptions of DAC/TCG and DGS. You'll have to look at AWS D1.1. for instance for knowledge of their sensitivity setting requirements. Knowledge of these techniques is desirable but will such knowledge really improve your inspection method? You use DAC because the Codes and standards you work to require you to assess indications to those DAC's. A report that a reflector was 3,5 mm equivalent FBH size to DGS would most probably be rejected.

DGS-If you have a signal feom a flaw at a certain depth, you can compare the signal of BWE from the FBH at that depth. The defect then could be sized as equivalent of the size of the FBH.

Size 0.24

Size 0.24

2.4depth

http://www.ndt.net/article/berke/berke_e.htm

Locating & Sizing Flaws

Locating reflectors with an angle-beam probe Fig. 53 Scanning a reflector using an angle beam probe The echo of a discontinuity on the instrument display does not now give us any direct information about its position in the material. The only available information for determination of the reflector position is the scale position and therefore the sound path s, this means the distance of the discontinuity from the index point (sound exit point) of the probe, Fig. 53. The mathematics of the right-angled triangle helps us to evaluate the Surface Distance and the Depth of a reflector which are both important for the ultrasonic test, Fig. 54a. We therefore now have the possibility to instantly mark a detected flaw's position on the surface of the test object by measurement of the surface distance from the sound exit point and to give the depth. For practical reasons, the reduced surface distance is used because this is measured from the front edge of the probe. The difference between the surface distance and the reduced surface distance corresponds to the x-value of the probe, this is the distance of the sound exit point to the front edge of the probe, Fig. 54b.

With ultrasonic instruments having digital echo evaluation these calculations are naturally carried out by an integrated microprocessor and immediately displayed so that the operator does not need to make any more timeconsuming calculations, Fig. 55. This is of great help with weld testing because with the calculation of the flaw depth an additional factor must be taken into account, namely: whether the sound pulses were reflected from the opposing wall. If this is the case then an apparent depth of the reflector is produced by using the depth formula which is greater than the thickness T of the test object. The ultrasonic operator must acertain whether a reflection comes from the opposite wall and then proceed with calculating the reflector depth, Fig. 56b.

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/common-test-practices/

Practice Makes Perfect 81. The 100 mm radius in an IIW block is used to: (a) Calibrate sensitivity level (b) Check resolution (c) Calibrate angle beam distance (d) Check beam angle 80. The 50 mm diameter hole in an IIW block is used to: (a) Determine the beam index point (b) Check resolution (c) Calibrate angle beam distance (d) Check beam angle

Practice Makes Perfect 35. The 2 mm wide notch in the IIW block is used to: (a) Determine beam index point (b) Check resolution (c) Calibrate angle beam distance (d) Check beam angle

Addendum-01b Equipment Calibration

My ASNT Level III UT Study Notes 2014-June.

Pulse-Echo Instrumentation

The Circuitry:       

Voltage activation of the PE crystal Ultrasound formation Propagation Reflection Charge formation of crystal Processing Display

Pulse-Echo Instrumentation Transmitter

TRX

Receiver Amplifier

Detector

Scan Converter Display

TGC

TGC – Time Gain Compensation Circuit

Pulse-Echo Instrumentation Pulser Components 1. HV pulse generator 2. The clock generator 3. The transducer

Pulse-Echo Instrumentation Generated Wave

Applied Voltage

+

+

V

P TIME

TIME

-

-

Pulse-Echo Instrumentation The Pulser rate is known as the pulse repetition frequency (PRF). Typical PRF 3,000 – 5,000. PRF automatically adjusted as a function of imaging depth.

Pulse-Echo Instrumentation Switch that controls the output power of the HV generator is the attenuator.

Pulse-Echo Instrumentation

TRX

PULSER

ATTENUATOR

Pulse-Echo Instrumentation CLOCK GENERATOR Controls the actual number of pulses which activate the crystal. Responsible for sending timing signal to the 1. Pulse generator 2. TGC circuitry 3. Memory

Pulse-Echo Instrumentation CLOCK GENERATOR

TGC UNIT

HV GENERATOR

MEMORY

TRS

TRX

CRT DISPLAY

Pulse-Echo Instrumentation Sensitivity refers to the weakest echo signal that the instrument is capable of detecting and displaying. Factors that determine sensitivity are 1. 2. 3. 4.

Transducer frequency Overall and TGC receiver gain Reject control Variable focal zone on array real-time instruments.

Pulse-Echo Instrumentation Increasing the voltage causes 1. Greater amplitude – greater penetration 2. Longer pulses – degrades axial resolution 3. Increase exposure

Pulse-Echo Instrumentation Transducer has dual roles; transmitting and receiving signals. The transducer is capable of handling a wide range of voltage amplitude. The Receiver is capable of handling only smaller signals Therefore it is desirable to isolate the pulser circuit from the receiver circuit.

Pulse-Echo Instrumentation The Transmit Receive Switch TRS – positioned at the input of the receiver and is designed to pass only voltages signals originating at the transducer by the returning echoes.

Pulse-Echo Instrumentation The Receiver Unit consist of 1. Radiofrequency Amplifier 2. Time gain compensation TGC unit 3. Demodulation Circuit 4. Detector Circuit 5. Video Amplifier

MEMORY PULSER

TRX

TGC UNIT

TRS

RF RECEIVER CRT DISPLAY DEMODULATOR

DETECTOR

VIDEO AMPLIFIER

Pulse-Echo Instrumentation Radio-Frequency Amplifier • Amplify weak voltage signals. • This is called GAIN

Pulse-Echo Instrumentation Electric signals generated by the transducer are weak and needs amplification. The gain is the ratio of the output to input Voltage or Power. Gain = Voltage Out Voltage In

Pulse-Echo Instrumentation The Imaging effect of adjusting gain are: 1. Increasing the gain - increased sensitivity, better penetration 2. Decreasing the gain – decreased sensitivity, less penetration 3. Too high a gain – overloads the display, loss or spatial resolution

Amplitude

Pulse-Echo Instrumentation

Saturation Level

Normal Gain

Distance

Pulse-Echo Instrumentation Excess Gain Amplitude

Saturation Level

Distance

Pulse-Echo Instrumentation Primary objective of grayscale pulse-echo imaging is to make all like reflectors appear the same in the Image regardless where they are located in the sound beam.

Pulse-Echo Instrumentation Time Gain Compensation TGC TGC - electronic process of adjusting the overall system gain as a function of the transmit time.

Pulse-Echo Instrumentation TGC Controls • Near Gain • Slope Delay • Slope • Knee • Far Gain • Body Wall

Pulse-Echo Instrumentation KNEE

Gain dB

NEAR GAIN

DELAY

SLOPE

Depth cm

MAX GAIN

Pulse-Echo Instrumentation KNEE

Gain dB

NEAR GAIN SLOPE

Depth cm Body wall

MAX GAIN

Pulse-Echo Instrumentation KNEE

Gain dB

SLOPE CUT-OFF

DELAY

Depth cm

Pulse-Echo Instrumentation The slide potentiometer allows adjustment of receiver gain for small discrete depth increments.

Pulse-Echo Instrumentation Slide Potentiometer

Gain dB

Depth (Time)

Pulse-Echo Instrumentation Frequency Tuning of the Receiver The frequency band width of the receiver refers to the range of ultrasound signal frequencies that the receiver can amplify with a maximum gain.

Pulse-Echo Instrumentation Types of Amplifiers • Wide-Band • Narrow-Band

Pulse-Echo Instrumentation Wide-band amplifier

Narrow-band amplifier

Gain

Gain

Frequency MHz

Frequency MHz

Pulse-Echo Instrumentation Receiver Unit

Receiver A

Receiver B Output To System

TRX Receiver C

Frequency Selector Switch

Receiver D

Pulse-Echo Instrumentation DYNAMIC RANGE The dynamic range is a measure of the range of echo signal amplitudes. The dynamic range can be measured at any point. The dynamic range decreases from transducer, to receiver to scan converter and finally to display.

Pulse-Echo Instrumentation Large range in signal amplitudes is due to: 1. Normal variation in the reflection amplitude. 2. Frequency dependent tissue attenuation.

Pulse-Echo Instrumentation RF amplifier can handle a wide range of signal amplitude at its input – but cannot accommodate the corresponding output using linear amplification.

Pulse-Echo Instrumentation Linear amplification - all voltages amplitudes, regardless of size at the point of input are amplified with the same gain factor.

Pulse-Echo Instrumentation LOGARITHMIC AMPLIFICATION In Logarithmic amplification weak echoes amplitudes are amplified more than strong echoes. This can reduced the dynamic range by as much as 50%. The process of reducing the signal DR by electronic means is called COMPRESSION

Pulse-Echo Instrumentation

A

Linear Amplification

Gain

B Logarithmic Amplification

Input signal

Pulse-Echo Instrumentation R-F amplifier can also set the electronic level in the machine. S-N level – compares real echo signals the system can handle versus the non-echo signals presents (Noise). The Higher the SN ratio – better the operation of the system.

Pulse-Echo Instrumentation Pre-amplification is a technique to reduce system noise. Positioning of part of the amplifier circuitry in the transducer housing reduces system noise.

Pulse-Echo Instrumentation REJECTION Rejection is the receiver function that enables the operator to systematically increase or decrease the minimum echo signal amplitude which can be displayed. Alternate names = Threshold, Suppression.

Pulse-Echo Instrumentation Saturation Level

Rejection Level Dynamic Range Noise Level Zero Signal Level

Pulse-Echo Instrumentation SIGNAL PROCESSING RF waveform – oscillating type of voltage signal (AC) First Step in processing the signal is Demodulation. Demodulation is the process of converting the electric signal from one form to another.

Pulse-Echo Instrumentation DEMODULATION  Rectification  Detection

Pulse-Echo Instrumentation RECTIFICATION • Rectification results in the elimination of the negative portion of the RF signals •

Half Wave Rectification

•

Full wave Rectification

Pulse-Echo Instrumentation Half-Wave Rectification

Pulse-Echo Instrumentation Full-Wave Rectification

Pulse-Echo Instrumentation DETECTION The main effect of detecting the rectified RF signal is to round out or smooth the signal as to have a single broad peak. The rectified RF signal following detection is referred to as a Video Signal.

Pulse-Echo Instrumentation Smoothing

Pulse-Echo Instrumentation The video signal is then further amplified by the VIDEO AMPLIFIER. The output from the video amplifier is forwarded to 1. CRT or 2. Scan converter

Pulse-Echo Instrumentation DIGITAL SCAN CONVERTER The device that stores the echo signal is called a Scan converter.

Pulse-Echo Instrumentation All Scan Converters are designed to 1. Store echoes in appropriate location 2. Encode echoes in shade of gray 3. Read out echoes in a horizontal raster format

Pulse-Echo Instrumentation 4. Digital Memory is divided into small squares = Pixel. 5. The Pixels form the Image Matrix 6. Total # of storage location = rows x columns 7. x and y location = ADDRESS

Matrix

Rows x, coordinates

Matrix

Columns, y coordinates

Matrix Pixel

10x 10y

X, Y ADDRESS 8x 7y

5x 5y

3x 3y

1x 1y

Pulse-Echo Instrumentation In the Scan converter the echoes are processed on a firstcome first-in basis.

X X X

X

X X

X X X

X X X

X X X

X

X X

X X X

X X X

50

50

50

50

50

50

50

50 50

50 50

50

Raster Process

50

50

50

50

50

50

50

50 50

50 50

50

Pulse-Echo Instrumentation DIGITAL SCAN CONVERTER • Convert echo voltage signal into a numerical value. •

Each numerical value corresponds to a shade of gray.

Pulse-Echo Instrumentation The number of shades of gray is determined by the BIT CAPACITY. # of shades of gray = 2

Pulse-Echo Instrumentation Echoes dB

Pulse-Echo Instrumentation Bit

Shades of Gray

1

2

2

4

3

8

4

16

5

32

6

64

7

128

8

256

Pulse-Echo Instrumentation Gray Scale Resolution = dynamic range (dB) # of gray shades

Pulse-Echo Instrumentation Operator can select different A/D conversion scheme (Preprocessing). Each preprocessing curve is called an algorithm and assigns a specific percentage amount of shades of gray to regions of the echo amplitude.

Pulse-Echo Instrumentation % Available Shade of gray

100% 1 2 50% 3

4

0% Echo Strength

Pulse-Echo Instrumentation POST PROCESSING Assignment of specific display brightness to numerical echo amplitudes read out of the digital memory.

Pulse-Echo Instrumentation 9

7

8

8

8

8

8

8

8

9

8

7

8

8

8

8

7

8

8

9

8

8

8

8

SMOOTHING

Pulse-Echo Instrumentation The DSC is not necessary for image display, but is needed for the following post-processing functions. • Video Invert • Display Invert • Display Subdivision • Zoom Magnification

Pulse-Echo Instrumentation Zoom Magnification • Read Zoom • Write Zoom

Pulse-Echo Instrumentation Resolution at the DSC 1. Find Matrix size 2. Determine FOV ( width/length) 3. Calculate pixels/cm 4. Find linear distance/pixel = resolution

Pulse-Echo Instrumentation Data PreProcessing

ADC

Echo Signal

Data Collection & Formatting

M A R

Data PostProcessing

Data Reformatting

Positional Data

Display

Pulse-Echo Instrumentation 1. ROM 2. PROM 3. RAM

65. In Figure 3, transducer A is being used to establish: A. B. C. D.

Verification of wedge angle Sensitivity calibration Resolution An index point

66. In Figure 3, transducer C is being used to check: A. B. C. D.

Distance calibration Resolution Sensitivity calibration Verification of wedge angle

67. In Figure 3, transducer D is being used to check: A. B. C. D.

Sensitivity calibration Distance calibration Resolution Verification of wedge angle

68. When the incident angle is chosen to be between the first and second critical angles, the ultrasonic wave generated within the part will be: A. B. C. D.

Longitudinal Shear Surface Lamb

69. In Figure 4, transducer B is being used to check: A. B. C. D.

The verification of wedge angle Resolution Sensitivity calibration Distance calibration

Q: In a UT test system where signal amplitudes are displayed on a CRT, an advantage of a frequency-independent attenuator over a continuously variable gain control is that: A. the pulse shape distortion is less B. the signal amplitude measured using the attenuator is independent of frequency C. the dynamic range of the system is decreased D. the effect of amplification threshold is avoided Q: An amplifier in which received echo pulses must exceed a certain threshold voltage before they can be indicated might be used to: A. suppress amplifier noise, unimportant scatter echoes, or small flaw echoes which are of no consequence B. provide a screen display with nearly ideal vertical linearity characteristics C. compensate for the unavoidable effects of material attenuation loss D. provide distance amplitude correction automatically

Q: The output voltage from a saturated amplifier is: A) 180 degrees out of phase from the input voltage B) lower than the input voltage C) nonlinear with respect to the input voltage D) below saturation Q: The transmitted pulse at the output of the pulser usually has a voltage of 100 to 1000V, whereas the voltages of the echo at the input of the amplifier are on the order of: A) 10 Volts B) 50 Volts C) .001 to 1 Volts D) 1 to 5 Volts

Q: The intended purpose of the adjustable calibrated attenuator of a UT instrument is to: A) control transducer dampening B) increase the dynamic range of the instrument C) broaden the frequency range D) attenuate the voltage applied to the transducer

Addendum-02 Equations & Calculations

My ASNT Level III UT Study Notes 2014-June.

Trigonometry

http://www.mathwarehouse.com/trigonometry/sine-cosine-tangent.php

Contents: 1. Material Acoustic Properties 2. Ultrasonic Formula 3. Properties of Acoustic Wave 4. Speed of Sound 5. Attenuation 6. What id dB 7. Acoustic Impedance 8. Snell’s Law 9. S/N Ratio 10. Near / Far Field 11. Focusing & Focal Length 12. Offsetting for Circular Specimen 13. Quality “Q” Factors 14. Inverse Law & Inverse Square Law

http://www.ndt-ed.org/GeneralResources/Calculator/calculator.htm

1.0

Material Acoustic Properties

Material

Logitudinal wave

Shear wave

mm/μs

mm/μs

Z Acoustic Impedence

Acrylic resin (Perspex)

2.74

1.44

3.23

Steel - SS 300 Series

5.613

3.048

44.6

Steel - SS 400 Series

5.385

2.997

41.3

Steel 1020

5.893

3.251

45.4

Steel 4340

5.842

3.251

45.6

http://www.ndtcalc.com/utvelocity.html

2.0

Ultrasonic Formula

http://www.ndt-ed.org/GeneralResources/Calculator/calculator.htm

Ultrasonic Formula

Ultrasonic Formula

α = Transducer radius

3.0

Properties of Acoustic Plane Wave

Wavelength, Frequency and Velocity Among the properties of waves propagating in isotropic solid materials are wavelength, frequency, and velocity. The wavelength is directly proportional to the velocity of the wave and inversely proportional to the frequency of the wave. This relationship is shown by the following equation.

4.0

The Speed of Sound

Hooke's Law, when used along with Newton's Second Law, can explain a few things about the speed of sound. The speed of sound within a material is a function of the properties of the material and is independent of the amplitude of the sound wave. Newton's Second Law says that the force applied to a particle will be balanced by the particle's mass and the acceleration of the the particle. Mathematically, Newton's Second Law is written as F = ma. Hooke's Law then says that this force will be balanced by a force in the opposite direction that is dependent on the amount of displacement and the spring constant (F = -kx). Therefore, since the applied force and the restoring force are equal, ma = -kx can be written. The negative sign indicates that the force is in the opposite direction.

F= ma = -kx

What properties of material affect its speed of sound? Of course, sound does travel at different speeds in different materials. This is because the (1) mass of the atomic particles and the (2) spring constants are different for different materials. The mass of the particles is related to the density of the material, and the spring constant is related to the elastic constants of a material. The general relationship between the speed of sound in a solid and its density and elastic constants is given by the following equation:

V is the speed of sound Eleatic constant → spring constants

Density → mass of the atomic particles

Where V is the speed of sound, C is the elastic constant, and p is the material density. This equation may take a number of different forms depending on the type of wave (longitudinal or shear) and which of the elastic constants that are used. The typical elastic constants of a materials include:  Young's Modulus, E: a proportionality constant between uniaxial stress and strain.  Poisson's Ratio, n: the ratio of radial strain to axial strain  Bulk modulus, K: a measure of the incompressibility of a body subjected to hydrostatic pressure.  Shear Modulus, G: also called rigidity, a measure of a substance's resistance to shear.  Lame's Constants, l and m: material constants that are derived from Young's Modulus and Poisson's Ratio.

E/N/G

5.0

Attenuation

The amplitude change of a decaying plane wave can be expressed as:

In this expression Ao is the unattenuated amplitude of the propagating wave at some location. The amplitude A is the reduced amplitude after the wave has traveled a distance z from that initial location. The quantity α is the attenuation coefficient of the wave traveling in the z-direction. The α dimensions of are nepers/length, where a neper is a dimensionless quantity. The term e is the exponential (or Napier's constant) which is equal to approximately 2.71828.

http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm

Spreading/ Scattering/ adsorption (reflection is a form of scaterring) Adsoprtion

Scaterring

Spreading

Scaterrring

Attenuation can be determined by evaluating the multiple backwall reflections seen in a typical A-scan display like the one shown in the image at the bottom. The number of decibels between two adjacent signals is measured and this value is divided by the time interval between them. This calculation produces a attenuation coefficient in decibels per unit time Ut. This value can be converted to nepers/length by the following equation.

Where v is the velocity of sound in meters per second and Ut is in decibels per second.

Amplitude at distance Z

Where v is the velocity of sound in meters per second and Ut is in decibels per second (attenuation coefficient). α is the attenuation coefficient of the wave traveling in the z-direction. The α dimensions of are nepers/length (nepers constant).

Attenuation is generally proportional to the square of sound frequency. Quoted values of attenuation are often given for a single frequency, or an attenuation value averaged over many frequencies may be given. Also, the actual value of the attenuation coefficient for a given material is highly dependent on the way in which the material was manufactured. Thus, quoted values of attenuation only give a rough indication of the attenuation and should not be automatically trusted. Generally, a reliable value of attenuation can only be obtained by determining the attenuation experimentally for the particular material being used.

Attenuation ∝ Frequency2 (f )2

Which Ut?

U0t , A0o U1t , A1o , α1 1

1

7.0

Acoustic Impedance

Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid. The acoustic impedance (Z) of a material is defined as the product of its density (p) and acoustic velocity (V).

Z = pV Acoustic impedance is important in: 1. the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances. 2. the design of ultrasonic transducers. 3. assessing absorption of sound in a medium.

The following applet can be used to calculate the acoustic impedance for any material, so long as its density (p) and acoustic velocity (V) are known. The applet also shows how a change in the impedance affects the amount of acoustic energy that is reflected and transmitted. The values of the reflected and transmitted energy are the fractional amounts of the total energy incident on the interface. Note that the fractional amount of transmitted sound energy plus the fractional amount of reflected sound energy equals one. The calculation used to arrive at these values will be discussed on the next page.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/applet_2_6/applet_2_6.htm

Reflection/Transmission Energy as a function of Z

Reflection and Transmission Coefficients (Pressure)  This difference in Z is commonly referred to as the impedance mismatch.  The value produced is known as the reflection coefficient. Multiplying the reflection coefficient by 100 yields the amount of energy reflected as a percentage of the original energy.  the transmission coefficient is calculated by simply subtracting the reflection coefficient from one. Ipedence mismatch

Reflection coefficient

Using the above applet, note that the energy reflected at a water-stainless steel interface is 0.88 or 88%. The amount of energy transmitted into the second material is 0.12 or 12%. The amount of reflection and transmission energy in dB terms are -1.1 dB and -18.2 dB respectively. The negative sign indicates that individually, the amount of reflected and transmitted energy is smaller than the incident energy.

If reflection and transmission at interfaces is followed through the component, only a small percentage of the original energy makes it back to the transducer, even when loss by attenuation is ignored. For example, consider an immersion inspection of a steel block. The sound energy leaves the transducer, travels through the water, encounters the front surface of the steel, encounters the back surface of the steel and reflects back through the front surface on its way back to the transducer. At the water steel interface (front surface), 12% of the energy is transmitted. At the back surface, 88% of the 12% that made it through the front surface is reflected. This is 10.6% of the intensity of the initial incident wave. As the wave exits the part back through the front surface, only 12% of 10.6 or 1.3% of the original energy is transmitted back to the transducer.

Practice Makes Perfect Following are the data:

Q1: What is the percentage of initial incident sound wave that will reflected from the water/Aluminum interface when the sound first enter Aluminum?

R= (Z1-Z2)2 / (Z1+Z2)2 = (0.149-1.72)2/(0.149+1.72)2 R= 0.707, Answer= 70.7%

Q2: What is the percentage of sound energy that will finally reenter the water after reflected from the backwall of Aluminum? (Do not consider material attenuation and other factors) Answer: 6%

0.706 – initial Back wall

0.2934

0.207x 0.2934=0.0609 Second Backwall echo 0.2934x 0.706 = 0.207

8.0

Snell’s Law

Snell's Law holds true for shear waves as well as longitudinal waves and can be written as follows

= Where: VL1 is the longitudinal wave velocity in material 1. VL2 is the longitudinal wave velocity in material 2. VS1 is the shear wave velocity in material 1. VS2 is the shear wave velocity in material 2.

Snell’s Law

http://education-portal.com/academy/lesson/refraction-dispersion-definition-snells-law-index-of-refraction.html#lesson

Practice Makes Perfect 5. For an ultrasonic beam with normal incidence, the reflection coefficient is given by: (a) [(Z1+Z2)2]/[(Z1-Z2)2] (b) (Z1+Z2)/(Z1-Z2) (c) [(4) (Z1)(Z2)]/[(Z1+Z2)2] (d) [(Z1-Z2)2]/[Z1+Z2)2] 6. For an ultrasonic beam with normal incidence the transmission coefficient is given by: (a) [(Z1+Z2)2]/[(Z1-Z2)2] (b) (Z1+Z2)/(Z1-Z2) (c) [(4) (Z1)(Z2)]/[(Z1+Z2)2] (d) [(Z1-Z2)2]/[Z1+Z2)2]

Practice Made Perfect 7. Snell's law is given by which of the following: (a) (Sin A)/(Sin B) = VB/VA (b) (Sin A)/(Sin B) = VA/VB (c) (Sin A)/ VB = V(Sin B)/VA (d) (Sin A)[VA] = (Sin B)[ VB] 8. Snell's law is used to calculate: (a) Angle of beam divergence (b) Angle of diffraction (c) Angle of refraction (d) None of the above

Practice Makes Perfect 9. Calculate the refracted shear wave angle in steel [VS = 0.323cm/microsec] for an incident longitudinal wave of 37.9 degrees in Plexiglas [VL = 0.267cm/ microsec] (a) 26 degrees (b) 45 degrees (c) 48 degrees (d) 64 degrees 10. Calculate the refracted shear wave angle in steel [VS = 0.323cm/microsec] for an incident longitudinal wave of 45.7 degrees in Plexiglas [VL = 0.267cm/ microsec] (a) 64 degrees (b) 45.7 degrees (c) 60 degrees (d) 70 degrees

Practice Makes Perfect 11. Calculate the refracted shear wave angle in aluminium [VS = 0.31cm/ microsec] for an incident longitudinal wave of 43.5 degrees in Plexiglas [VL = 0.267cm/microsec] (a) 53 degrees (b) 61 degrees (c) 42 degrees (d) 68 degrees 12. Calculate the refracted shear wave angle in aluminium [VS = 0.31cm/microsec] for an incident longitudinal wave of 53 degrees in Plexiglas [VL = 0.267cm/microsec] (a) 53 degrees (b) 61 degrees (c) 42 degrees (d) 68 degrees

9.0

S/N Ratio

The following formula relates some of the variables affecting the signal-tonoise ratio (S/N) of a defect:

FOM: Factor of merits at center frequency

The following formula relates some of the variables affecting the signal-tonoise ratio (S/N) of a defect:

Sound Volume: Area x pulse length Δt

Material properties Flaw geometry at center frequency: Figure of merit FOM and amplitudes responds

10.

Near/ Far Fields

http://miac.unibas.ch/PMI/05-UltrasoundImaging.html

where α is the radius of the transducer and λ the wavelength.

For beam edges at null condition K=1.22

Modified Near Zone

T Perspex

Modified Zf

Example: Calculate the modified Near Zone for; • 5 MHz shear wave transducer • 10mm crystal • 10 mm perspex wedge Perspex L-wave: 2730 m/s Steel S-wave: 3250 m/s Steel L-wave: 5900 m/s Modified NZ= (0.012 x f) / (4v) – 0.01(2730/3250) =0.0300m = 30mm

Apparent Near Zone distance

11.0

Focusing & Focal Length

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/beam-characteristics/

The focal length F is determined by following equation;

Where: F = Focal Length in water R = Curvature of the focusing len n = Ration of L-velocity of epoxy to L-velocity of water

12.0

Offset of Normal probe above circular object V1

θ1

θ1

R

θ2 V2

Calculate the offset for following conditions: Aluminum rod being examined is 6" diameter, what is the off set needed for (a) 45 refracted shear wave (b) Longitudinal wave to be generated? (L-wave velocity for AL=6.3x105cm/s, T-wave velocity for AL=3.1x105 cm/s, Wave velocity in water=1.5X105 cm/s) Question (a)

Question (b)

13.0

“Q” Factor

3dB down

14.0

Inverse Law and Inverse Square Law

For a small reflector where the size of reflector is smaller than the beam width, the echoes intensity from the same reflector varies inversely to the square of the distance.

5cm

75% FSH

7.5cm

33% FSH

Inverse Square Law

http://www.cyberphysics.co.uk/general_pages/inverse_square/inverse_square.htm

Inverse Law: For large reflector, reflector greater than the beam width, e.g. backwall echoes from the same reflector at different depth; the reflected signal amplitude varies inversely with the distance.

10cm

7.5cm

DGS Distance Gain Sizing

Y-axis shows the Gain

size of reflector is given as a ratio between the size of the disc and the size of the crystal.

X-axis shows the Distance from the probe in # of Near Field

– Distance Gain Size is a method of setting sensitivity or assessing the signal from an unknown reflector based on the theoretical response of a flatbottomed hole reflector perpendicular to the beam axis. (DGS does not size the flaw, but relate it with a equivalent reflector) The DGS system was introduced by Krautkramer in 1958 and is referred to in German as AVG. A schematic of a general DGS diagram is shown in the Figure. The Y-axis shows the Gain and X-axis shows the Distance from the probe. In a general DGS diagram the distance is shown in units of Near Field and the scale is logarithmic to cover a wide range.

The blue curves plotted show how the amplitudes obtained from different sizes of disc shaped reflector (equivalent to a FBH) decrease as the distance between the probe and the reflector increases.

In the general diagram the size of reflector is given as a ratio between the size of the disc and the size of the crystal. The red curve shows the response of a backwall reflection. The ratio of the backwall to the crystal is infinity (∞). Specific DGS curves for individual probes can be produced and so both the distance axis and the reflector sizes can be in mm. If the sensitivity for an inspection is specified to be a disc reflector of a given size, the sensitivity can be set by putting the reflection from the backwall of a calibration block or component to the stated %FSH. The gain to be added can be then obtained by the difference on the Y-axis between the backwall curve at the backwall range and the curve of the disc reflector of the given size at the test range. If the ranges of the backwall and the disc reflector are different, then attenuation shall be accounted for separately. Alternatively, the curves can be used to find the size of the disc shaped reflector which would give the same size echo as a response seen in the flaw detector screen.

20-4dB=16dB (deduced) Δ Flaw =30-16=14dB

Data: Probe frequency: 5MHz Diameter: 10mm compression probe Plate thickness: 100mm steel Defect depth: 60mm deep Gain for flaw to FSH: 30dB BWE at 100mm: 20dB

20dB (measured)

Example: If you has a signal at a certain depth, you can compare the signal of the flaw to what the back wall echo (BWE) from the same depth and estimate the FBH that would give such a signal at the same depth. The defect can then be size according to a FBH equivalent. Data: Probe frequency: 5MHz Diameter: 10mm compression probe Plate thickness: 100mm steel Defect depth: 60mm deep Gain for flaw to FSH: 30dB BWE at 100mm: 20dB ------------------------------------------------------------------------Near field: 21mm, flaw location= 3xNear Field From the chart BWE at 60mm will be 20-4dB=16dB Flaw signal Gain is 30dB-16dB= 14dB Used the flaw signal Gain and locate the equivalent reflector size is between 0.4 to 0.48 of the probe diameter, say 0.44 x10mm = 4.4mm equivalent reflector size.

http://www.olympus-ims.com/en/atlas/dgs/

More on DGS/AVG by Olympus http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/dgs-avg/

DGS is a sizing technique that relates the amplitude of the echo from a reflector to that of a flat bottom hole at the same depth or distance. This is known as Equivalent Reflector Size or ERS. DGS is an acronym for DistanceGain-Size and is also known as AVG from its German name, Abstand Verstarkung Grosse. Traditionally this technique involved manually comparing echo amplitudes with printed curves, however contemporary digital flaw detectors can draw the curves following a calibration routine and automatically calculate the ERS of a gated peak. The generated curves are derived from the calculated beam spreading pattern of a given transducer, based on its frequency and element diameter using a single calibration point. Material attenuation and coupling variation in the calibration block and test specimen can be accounted for.

DGS is a primarily mathematical technique originally based on the ratio of a circular probe’s calculated beam profile and measurable material properties to circular disk reflectors. The technique has since been further applied to square element and even dual element probes, although for the latter, curve sets are empirically derived. It is always up to the user to determine how the resultant DGS calculations relate to actual flaws in real test pieces. An example of a typical DGS curve set is seen below. The uppermost curve represents the relative amplitude of the echo from a flat plate reflector in decibels, plotted at various distances from the transducer, and the curves below represent the relative amplitude of echoes from progressively smaller disk reflectors over the same distance scale.

As implemented in contemporary digital flaw detectors, DGS curves are typically plotted based on a reference calibration off a known target such as a backwall reflector or a flat bottom hole at a given depth. From that one calibration point, an entire curve set can be drawn based on probe and material characteristics. Rather than plotting the entire curve set, instruments will typically display one curve based on a selected reflector size (registration level) that can be adjusted by the user. In the example below, the upper curve represents the DGS plot for a 2 mm disk reflector at depths from 10 mm to 50 mm. The lower curve is a reference that has been plotted 6 dB lower. In the screen at left (figure 1), the red gate marks the reflection from a 2 mm diameter flat bottom hole at approximately 20 mm depth. Since this reflector equals the selected registration level, the peak matches the curve at that depth. In the screen at right (Figure 2), a different reflector at a depth of approximately 26 mm has been gated. Based on its height and depth in relation to the curve the instrument calculated an ERS of 1.5 mm.

Figure1:

Figure2:

15.0

Pulse Repetitive Frequency/Rate and Maximum Testable Thickness

Clock interval = 1/PRR Maximum testable length = ½ x Velocity x Clock interval Note: The Clock interval has neglected the time occupied by each pulse.

16.0

Immersion Testing of Circular Rod

Q4-12 Answer: First calculate the principle offset d; ϴ = Sin-1(1483/3250 xSin45)=18.8 ° d=R.Sin18.8= 0.323 (Assume R=1). Wobbling ±10%; d’=0.355 ~ 0.290 d’=0.355, ϴ = Sin-1(0.355)=20.8 ° giving inspection Φ = Sin-1(3250/1483xSin20.8)=51, 13.3% above 45 ° d’=0.290, ϴ = Sin-1(0.290)=16.9 ° giving inspection Φ = Sin-1(3250/1483xSin16.9)=39.6, 12% below 45 °

Maximum ϴ

ϴ max = Sin-1 (ID/OD)

Addendum-03 Questions & Answers Collection of My Pitfalls

Uncertain Questions 21. Which type of calibration block is used to determine the resolution of angle beam transducers per requirements of AWS and AASHTO a. b. c. d.

An IIW block A DSC block A rompus block An RC block

24. Resonance or standing waves are a result of: a. b. c. d.

mode conversion interference from reflected waves beam divergence (spread) attenuation of the sound waves

Make mistakes now, not during exam!

RC- Resolution Calibration Block

30. On an A-scan display the dead zone refers to: a. the distance contained within the near field (incorrect) b. the area outside the beam spread c. the distance covered by the front surface pulse width and recovery time d. the area between the near field and the far field 40. The second critical angle is the angle of the incident beam at which: a. b. c. d.

the angle of the refracted compression wave is 900 the angle of the reflected compression wave is 90° total reflection occurs surface waves are produced

--------------------------------------------------------------------------------

17. Surface waves are used to detect discontinuities in the test materials: a. b. c. d.

At half the depth. Above the lower surface. On the surface where the probe is in contact. None of the above.

26. Which of the following probes is most commonly used for testing welded metals for laminations before angle beam inspection. a. b. c. d.

Surface wave probe. Twin crystal 0° probe. Single crystal probe. An angle probe.

29. Artificial flaws can be produced by using: Side drilled holes Flat bottom holes EDM notches (http://www.phtool.com/pages/edm.asp) All of the above

31. As the acoustic impedance ratio between two materials approaches 1 the amount of sound reflected at an interface: a. b. c. d.

increases. decreases. is not affected. varies depending upon the velocity of the materials.

34. Significant errors in ultrasonic thickness measurements can occur if; a. Test frequency is varying at a constant rate. b. The velocity of propagation deviates substantially from an assumed constant value for a given material. c. Water is employed as a couplant between the transducer and the part being measured. d. None of the above should cause errors.

45. When examining thin materials for planar discontinuities oriented parallel to the part surface, what testing method is most often used: a. b. c. d.

Angle beam Through-transmission Straight beam - single crystal Straight beam - dual crystal

7. The ultrasonic test method in which finger damping in most effective in locating a discontinuity is: a. b. c. d.

shear wave longitudinal wave surface wave compressional wave

15. Which type of test block is used to check horizontal linearity and the dB accuracy per requirements of AWS and AASHTO? a. b. c. d.

Distance/Sensitivity block A DSC block A rompus block A shear wave calibration block

Mistake Made -------------------------------------------------------------------------------Question: Which probe will be used for critical examination in a forged component with a curved surface.: Your answer: 1 megahertz, 10mm dia. Correct answer: 10 megahertz, 25mm dia. Question: A general term applied to all cracks, inclusions, blow holes etc, which cause a reflection of sonic energy is: Your answer: a refractor Correct answer: a discontinuity Question: On an A-scan display the dead zone refers to: Your answer: the distance contained within the near field Correct answer: the distance covered by the front surface pulse width and recovery time

Mistake Made -------------------------------------------------------------------------------Question: Dead zone size depends on: Your answer: construction of the probe. Correct answer: All of the above. Question: The second critical angle is the angle of the incident beam at which: Your answer: total reflection occurs Correct answer: surface waves are produced ---------------------------------------------------------------------------------

Mistake Made -------------------------------------------------------------------------------Question: When a longitudinal wave encounters an interface between two material with different accoustic impedances, what occurs when the Your answer: Reflection and refraction Correct answer: Reflection Question: In an ultrasonic instrument, the number of pulses produced by an instrument in a given period of time in known as the:Your answer: pulse length of the instrument Correct answer: pulse repetition rate Question: Which probe will be used for critical examination in a forged component with a curved surface.:Your answer: 10 megahertz, 10mm dia.Correct answer: 10 megahertz, 25mm dia.

Question: Which type of screen presentation displays a profile or crosssectional view of the test specimen? Your answer: A-scan Correct answer: B-scan Question: When a longitudinal wave encounters an interface between two material with different accoustic impedances, what occurs when the Your answer: Refraction Correct answer: Reflection

Questions & Answers

Table 1.2

Chapter 1: Physical Principles Q1-10 The acoustic energy reflected at a plexiglass-quartz interface is equal to? Answer: R= (Z1-Z2)2 / (Z1+Z2)2 = (3.2-15.2)2 / (3.2+15.2)2= 42.53% Q1-11 The acoustic energy transmitted through a plexiglass-water interface is equal to? Answer: R= (Z1-Z2)2 / (Z1+Z2)2 = (3.2-1.5)2 / (3.2+1.5)2 = 13%, T= 1-R = 87% Q1-12 The first critical angle at a water-plexiglass interface will be? Answer: ϴ = Sin-1 (1483/2730) = 32.9°

Q1-13 The second critical angle at water-plexiglass interface will be? Answer: ϴ = Sin-1 (1483/1430) = Error! Q1-14 The incident angle need in immersion testing to develop a 70 shear wave in plexiglass is equal to? Answer: ϴ = Sin-1 (1483/1430 x sin70) = 77°

Q1-20 Two plate yield different back-wall reflections in pulse-echo testing (18dB) with their only apparent difference being in the second material void content. The plate are both 3” thick. What is the effective change in acoustic attenuation between the first and second plate? Answer: Sound path – 2 x thickness = 6” Attenuation = 18dB/6” = 3dB/in. Comment: The answer could be confused if the pulse-echo testing, 2-ways path length was not considered, arriving with the incorrect answer of 6dB/in

For evaluating material properties always remember to divide the result with the actual sweep distance if necessary! It was not a one-way–trip!

Q1-15 At a water-Aluminum interface, at an incident angle of 20°, the reflected and transmitted wave are? Answer: 60% transmitted and 40% reflected.

Q1-22 The beam spread half angle I the far field of a I” diameter transducer sending 5MHz longitudinal wave into Plexiglas block is? Answer: ϴ = Sin-1 (K λ/D) Assumed K=1.2 for null beam edge, ϴ = Sin-1 (K λ/D) =Sin-1(1.2V/DF)= Sin-1[1.2x2730x103/ (25.4x5x106)] =1.478° Q1-23 The near field of a round 1/2 “ diameter contact L-wave transducer being used on a steel test part operating at 3MHz is? Answer: Z= D2/4λ = 12.72 3x106 x / (4x5900x103) = 20.5mm

Chapter 2: Equipment Q2-5 A 5MHz 0.5” diameter flat search unit in water has a near field length of approximately? Answer: Z= D2/4λ = (12.72 x 5x106) / (4x 1480X103) = 136mm = 5.36” Q2-7 A 10MHz,0.5” diameter transducer placed on steel and acrylic in succession, the beam spread in these 2 material is? ϴ = sin-1(K λ/D). ϴFe = sin-1(1.2x5920x103/10x106x12.7) = 3.2°, ϴAcrylic = sin-1(1.2x2730x103/10x106x12.7) = 1.48°

Q2-12 An angle beam produce a 45° shear wave in steel, what is the incident angle? (Vs for steel=0.125in/ms, VL for plastid=0.105in/ms) Answer: Snell’s Law; ϴincident = Sin-1[(0.105/0.125) xSin45] = 36.43° Q2-13 Aluminum rod 6” diameter being examined in immersion technique, what is the required offset to generate a 45° refracted shear wave? Answer: First find the incident angle using Snell’s Law ϴincident = Sin-1[(1.5/3.1) xSin45] = 20° Offset = rSin20 = 3Sin20 = 1.026”

Q2-14 What is the offset required, if 45 refracted longitudinal wave to be generated? Answer: First find the incident angle using Snell’s Law ϴ incident = Sin-1[(1.5/6.3) xSin45] = 9.69° Offset = r.Sin9.69 ° = 3.Sin9.69 ° = 0.505” Q2-16 In a longitudinal wave immersion test of Titanium plate, an echoes pulse from an internal defect is observed 6.56μs following front echo. How deep is the defect below the front surface? Answer: Sound path travel= 6100000 x 6.56 x 10-6 = 40mm The actual depth = sound path / 2 = 20mm

Q2-17 A change in echo amplitude from 20% of FSH to 40% of FSH is a change of how many dB? Answer: ΔdB= 20log(20/40) = 6dB drop or -6dB. Q2-20 What is the lens radius of curvature is needed in order to have a 20mm diameter 5MHz transducer focus in water at a distance of 40mm drom the lens face? Answer: R=F(n-1/n), n= V Lens/V water , n= 2.67/1.49= 1.792. R=40(0.792/1.792) = 17.7mm

Q2-18 In Fig.29 what is the rate of attenuation in dB/in of 5MHz transducer in Far Field, the horizontal scale is 0.5” per division and the vertical scale is linear. Answer: ΔI = 20log(1.25/2) D= L=4.876m Will resolve the problem.

Q4-17 Illustrations

Complete loop=4.876m

Length of axial 8’ or 2.438m

The previous pulse return position when 2nd (next) pulse start to send

Incoming & returning wave meet

2nd pulse generating

0.958m

0.958m 0.522m

₵

Q4-18 Answer:

8. When testing a 30 mm diameter, 500 mm long shaft from the flat end of the shaft using longitudinal waves from a 20 mm diameter 2 MHz probe, numerous signals are seen on the screen after 500 mm. These are: a) ghost images b) side wall echoes c) internal thread indications d) none of the above

Break!

mms://a588.l3944020587.c39440.g.lm.akamaistream. net/D/588/39440/v0001/reflector:20587?BBCUID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5 d6448f55674c540f1856834&SSO2-UID=

Q5-20 Answer: None of above

Q5-22 Answer: Class C

Q5-22 Table B-1

5. At a solid to free boundary, an obliquely incident longitudinal wave from the solid can result in, at most: a) a reflected longitudinal wave only b) a reflected longitudinal and reflected shear wave c) a refracted longitudinal long wave d) a reflected longitudinal and reflected shear and refracted longitudinal wave 6. Geometric-optic treatment (?) of ultrasonic waves fails to account for: a) reflection b) refraction c) diffraction d) normal incidence 34.The most useful range of incident longitudinal wave angles for ultrasonic testing is: (a) Normal incidence to the first critical angle (b) First critical angle to the second critical angle (?) (c) Second critical angle to the third critical angle (d) Above the third critical angle

38. The angle of a refracted shear wave generated as a sound wave passes at an angle through an acoustic interface is dependant on: a) The acoustic impedances of the materials of each side of the interface b) The frequency of the incident sound wave c) The wavelength of the incident sound wave d) The hardness of the materials on each side of the interface 22. The three most common modes of sound vibration are: (a) Longitudinal, compressional, and transverse waves (b) Longitudinal, transverse and rayleigh waves (c) Transverse, longitudinal and shear waves (d) Transverse, shear waves and rayleigh waves

13. An oscilloscope display in which the screen base line is adjusted to represent the one way distance in a test piece is called a: (a) A scan display (b) B scan display (c) C scan display (d) D scan display 12. Which of the following test frequencies would generally provide the best penetration in a 12 inch thick specimen of coarse-grained steel? (a) 1.0 MHz (b) 2.25 MHz (c) 5.0 MHz (d) 10 MHz (Incorrect – silly mistake)

48. A more highly damped transducer crystal results in: (a) Better resolution (b) Better sensitivity (mistake) (c) Lower sensitivity (d) Poorer resolution 6. The portion of a test piece which is represented by the CRT screen area from zero to the rightmost edge of the initial pulse is called: (a) The dead zone (mistake) (b) The near field (c) The near zone (d) The far zone

17. Transducer focal lengths are normally specified as: (a) Distance in steel (b) Distance in aluminium (c) Distance in air (d) Distance in water (mistake) 21. An advantage of using a ceramic transducer in search units is that: (a) It is one of the most efficient generators of ultrasonic energy (b) It is one of the most efficient receivers of ultrasonic energy (c) It has a very low mechanical impedance (d) It can withstand temperatures as high as 700oC

47. When a vertical indication has reached the maximum signal height which can be displayed or viewed on the CRT of an ultrasonic instrument, the indication is said to have reached its: (a) Distance-amplitude height (mistake) (b) Absorption level (c) Vertical level (d) Limit of resolution

53. An ultrasonic instrument control which is used to adjust the sharpness of the CRT screen display is called: (a) Astigmatism or focus (b) Pulse repetition rate (c) Pulse energy (d) Gain

63. The purpose of the couplant is to: (a) Match impedances between the transducer and test piece (b) Absorb stray reflectors (c) Clean the test piece so a more efficient test may be continued (d) Lock the ultrasonic scanner into place prior to testing Note: by exclude the air between the 2 interfaces.

72. When conducting an immersion test, the water path distance must be controlled so that: a) Spurious signals are not created by surface waves on the test piece b) The (water path distance)/(diameter) ratio does not result in asymmetric standing waves c) The test piece discontinuity indications appear between the first front and first back surface echoes d) The second front surface echo does not appear on the CRT screen between the first front and first back surface echoes (?)

Immersion Testing Method

Standards Answer: C

Standards Answer: B

Standards Answer: A

Standards Answer: A (or C?)

Standards Answer: A

Standards Answer: C

Standards Answer: B

Standards Answer: C

Standards Answer: C

Standards Answer: A?

Arrows shown standard correct answers: Level I Q&A

Arrows shown standard correct answers: Level I Q&A

Study Blueeeeeeee… 28th July 2014 17:34

Arrows shown standard correct answers:

mms://a588.l3944020587.c39440.g.lm.akamaistre am.net/D/588/39440/v0001/reflector:20587?BBCUID=e5203c9d59fef1a79c12d8c601e839f58db16f7 d5d6448f55674c540f1856834&SSO2-UID=

Arrows shown standard correct answers: Level II Q&A

http://www.mtv123.com/mp3/45297/326534.shtml

Arrows shown standard correct answers:

Arrows shown standard correct answers:

R↑∝ F↑

Arrows shown standard correct answers:

Arrows shown standard correct answers:

Arrows shown standard correct answers:

3-Screen Height Linearity The ultrasonic testing instrument shall provide linear vertical presentation within ±5% (According to ASME Sec.V, Article 5 T-532) of the full screen height for 20% to 80% of the calibrated screen height. The procedure for evaluating screen height linearity is provided in appendix 1 of article 5, ASME code Sec.V and shall be performed at the beginning of each period of extended use (or every 3 months, which ever is less). http://www.inspection-for-industry.com/ultrasonic-testing.html

Take a break

mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/394 40/v0001/reflector:20587?BBCUID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f55674c5 40f1856834&SSO2-UID=

Calculation: Incident angle= 7° Refracted longitudinal wave = 29.11° Refracted shear wave = 15.49°

Arrows shown standard correct answers:

Arrows shown standard correct answers: Q2. During ultrasonic inspection of a weld, having a thickness of 28 mm angle beam search units are to be used. The recommended angle of search unit Is: a. 70º b. 60º c. 45º d. any one

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Ultrasonic Formula

Inverse Law and Inverse Square Law For a small reflector where the size of reflector is smaller than the beam width, the echoes intensity from the same reflector varies inversely to the square of the distance.

5cm

75% FSH

7.5cm

33% FSH

Inverse Square Law

http://www.cyberphysics.co.uk/general_pages/inverse_square/inverse_square.htm

Inverse Law: For large reflector, reflector greater than the beam width, e.g. backwall echoes from the same reflector at different depth; the reflected signal amplitude varies inversely with the distance.

10cm

7.5cm

Echo Amplitude- Reflector Size “D” & Depth “d” Relations: (small reflector- Inverse square law)

Amplitude α D2 Amplitude α 1/d2 Amplitude = kD2/d2 , k =constant Amplitude1/ Amplitude2 = D12 d22 / d12 D22

d

Amplitude D

Echo Amplitude- Reflector Size “D” & Depth “d” Relations: (large reflector- inverse law)

Amplitude α 1/d Amplitude = k/d , k =constant, Amplitude1/ Amplitude2 = d2 / d1

d

Amplitude D

Scanning Speed: Scanner speed = (PRR / Number of hits) × Effective diameter of probe Speed of test part = (PRR / Number of hits) × Effective diameter of probe Where: Effective dia. of probe = Dia. of probe – 2 [ (Dia. of probe) × (Percent of overlap between scan / 100) ] PRR = Pulse Repetition Rate Linear speed of disc or pipe in mm/ s = (2πr x RPM / 60) where r = radius of disc or pipe in mm RPM = Number of Rotation of pipe Per Minute = Revolution Per Minute